You can define a new command which changes the value of \arrayrulewidth
, draws the \cline
and then restores \arrayrulewidth
to its original value:
\documentclass{book}
\newlength{\Oldarrayrulewidth}
\newcommand{\Cline}[2]{%
\noalign{\global\setlength{\Oldarrayrulewidth}{\arrayrulewidth}}%
\noalign{\global\setlength{\arrayrulewidth}{#1}}\cline{#2}%
\noalign{\global\setlength{\arrayrulewidth}{\Oldarrayrulewidth}}}
\begin{document}
\begin{tabular}{ccc}
a & b & c \\\Cline{2pt}{2-3}
d & e & f \\\Cline{3pt}{1-2}
\end{tabular}
\end{document}

The first argument of \Cline
controls the "thickness" of the rule.
EDIT: The \cmidrule
command provided by the booktabs
package admits an optional argument controlling the thickness of the rule, so you can say:
\documentclass{book}
\usepackage{booktabs}
\begin{document}
\begin{tabular}{ccc}
a & b & c \\ \cmidrule[2pt]{2-3}
d & e & f \\ \cmidrule[3pt]{1-2}
\end{tabular}
\end{document}