Commercial Fixed Wing
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Distraction
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Flight Crew Returned To Clearance
Aircraft; Environment - Non Weather Related; Human Factors
Captain - PM. FO - PF (First PF leg in nearly 2 months - briefed as a threat during the brief).Weather conditions - Turbulent air and smoke causing low visibility and low level wind shear up to short final. Reported calm winds on the ground.ZZZ traffic was moderate and spacing was tight.ATC had given a clearance of 250 KIAS and a descent to 6;000 feet shortly after departing ZZZZZ on the ZZZZ4 arrival to join up for the ILS for [Runway] XXL.Inside of 15 DME from ZZZZZ1 ATC clearance was to slow to 170 KIAS and at or above 4;000 feet.Approaching the 90 degree turn at ZZZZZ1 ATC apologized for the late turn clearance and asked if we could still join the localizer for XXL; in which we responded we thought we could. Autopilot was engaged as the aircraft was approaching 4;000 feet while the PF turned heading to intercept the inbound course. The clearance was inbound course only yet the PF reached to engage both autopilots and arm approach mode. Instead of engaging the second autopilot the PF inadvertently depressed autopilot 2 button disengaging the autopilot all together.  PF announced 'autopilot disengaged; I have control.' This happened while in the turn and the aircraft was leveling. In the brief confusion of the autopilot disengagement as the crew sought to arm the LOC the PF allowed the aircraft to dip below 4;000 feet. The PM announced the altitude and the PF began to correct.  The speed brakes had been out during this time to assist with the 170KIAS request and at this time LS was creeping up.  There was a lag in the auto thrust to re-spool the engines for dipping below 4;000 feet so the PF went to TOGA thrust momentarily to initiate the spool up which sequenced the go-around function. Once LS and VS reversal was accomplished the PF moved the thrust levers to idle which disengaged the auto thrust and degrading automation even further. The PF announced that the auto thrust was disengaged. With the initial increase of power the aircraft climbed to roughly 4;500 feet.  All this took place in about 10-15 seconds.  This created a high level of initial confusion as the PM worked to reload the arrival and approach as well as respond to ATC to further slow to 160 KIAS and cleared ILS XXL. The crew worked together to rebuild the automation; silence the altitude aural warning; and re-engage the autopilot and autothrust systems while configuring the aircraft. All this was completed well before the FAF and even with shifting winds on final the remainder of the approach and landing was uneventful. Even though there wasn't a deviation from regulations and in fact ATC thanked us for our 'help' with the late intercept clearance and tight spacing there is plenty to learn from here.  Key take-always:Fatigue played a factor - prior to departing from ZZZ1 there was a ground delay for roughly 40 minutes while passengers were on the aircraft.  Weather departing ZZZ1 continued to play a factor with low level wind shear with thunderstorms in the vicinity.  The flight was a 5 hour leg to ZZZ.  Lacking proficiency played a factor - this was the first leg in which the FO was the PF since July. Responding reactively to ATC's requests which caused the crew to be rushed.Instinctively degrading automation instead of using automation to reduce work load by the PF. Lack of proficiency in rebuilding the automation by the PM.  However; good leadership by the Captain and good CRM by both crew members allowed for a successful outcome and a good learning.
[Report narrative contained no additional information.]
Air carrier flight crew reported an altitude deviation and cited fatigue and lack of flying as contributing factors.