#!latex

\documentclass[11pt,footinclude,headinclude]{scrreprt}

% use imakeidx instead of makeidx 
% until we investigate what's wrong with titlesec+komascr+... or format the Index heading properly
\usepackage{imakeidx}

\makeindex

\usepackage[linedheaders]{classicthesis} 
\usepackage{lipsum} 

\begin{document}
\tableofcontents

\chapter{Test Chapter} 
\lipsum[1-3] Some term defined\index{define}. 
\section{A Section} 
\lipsum[2-8] Some term\index{term} defined.

\addcontentsline{toc}{chapter}{\tocEntry{Index}}
\printindex

\end{document}