Q:

\chaptermark undefined error in classicthesis.sty when I (attempt to) redefine \spacedlowsmallcaps

My MWE does not compile and the error is: ! LaTeX Error: \chaptermark undefined.  I have debugged to the extent that I know the error is the interaction between classicthesis and my \SC command somehow interacting with \spacedlowsmallcaps -- which is why I am attempting to re-define the \spacedlowsmallcaps control sequence.
Basically I am trying to wrest control of smallcaps from classicthesis.
\documentclass[10pt,letterpaper]{scrartcl}
\usepackage{fontspec}
\setmainfont{Alegreya}

\newfontfamily{\SCaps}{Alegreya SC}
\DeclareTextFontCommand{\SC}{\SCaps}

\usepackage{classicthesis}
\renewcommand{\spacedlowsmallcaps}[1]{\DeclareRobustCommand{\SC{#1}}}

\begin{document}

Hello World

\spacedlowsmallcaps{Hello World}

\end{document}

A:

Of course you get an error: if you want to use scrartcl you have to call
\usepackage[nochapters]{classicthesis}

However your redefinition of \spacedlowsmallcaps makes no sense:
\renewcommand{\spacedlowsmallcaps}[1]{\SC{#1}}

is probably what you want.

There is no need to do tricks, though.
\documentclass[10pt,letterpaper]{scrartcl}

\usepackage{fontspec}

\usepackage[nochapters]{classicthesis}

\setmainfont{Alegreya}[
  SmallCapsFont=* SC,
]

\begin{document}

Hello World

\spacedlowsmallcaps{Hello World}

\end{document}

If you don't want to rely on soul for letter spacing when using XeLaTeX, add this code before \begin{document}:
\DeclareRobustCommand{\spacedlowsmallcaps}[1]{{%
  \normalfont\scshape
  \addfontfeatures{LetterSpace=10}%
  \MakeLowercase{#1}%
}}

