\documentstyle{report}

\begin{document}

\def\underscore{\leavevmode \kern.06em \vbox{\hrule width.3em}}
\let\myunderscore=_
\catcode`\_=\active \outer\def_{\underscore} % use \sb for subscript

\newenvironment{struct}%
{\begin{list}{}{\renewcommand{\makelabel}[1]{{\bf $\bullet$}{\tt ##1:}}}}%
{\end{list}}

\let\tmp=_
\let_=\myunderscore

\begin{struct}
\item[Wire\_list] can frep\_contain two types of records:
\end{struct}
\end{document}
