\documentclass[a4paper,12pt]{report}
\begin{document}
\newlength{\laenge}
\newcommand{\strich}[2][.5ex]
{\settowidth{\laenge}{#2}%
\makebox[0pt][l]{#2}%
\rule[#1]{\laenge}{0.15mm}}
.. was zur \strich{Durchstreichung} ...
.. \strich[-1pt]{Unterstreichen} ...
\end{document}
