Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Situational Awareness; Communication Breakdown
ATC Issue All Types; Conflict NMAC; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Human Factors
Level at 10000 ft. near ZABER intersection; ZABER 6 arrival into CLE. Traffic appeared on TCAS at 10 o'clock position and 400 ft. below. Received a traffic advisory from TCAS followed by a 'monitor vertical speed' followed by a resolution advisory to climb. PF (Pilot Flying) followed advisory and we climbed to 10;300 ft. Advisory cleared.  We returned to 10;000 ft. and I advised ATC of our deviation and a resolution advisory. Traffic was never reported to us by ATC. Traffic not in communication with ATC.
Air Carrier Captain reported responding to a RA; making evasive manuever; while level at 10;000 ft.