%%
%% The LaTeX Companion, 3ed
%%
%% Example 4-1-34 on page I-279 in "Styling description-like lists".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{159.0pt}

  \usepackage{microtype}  % improve typesetting in small columns

%StartShownPreambleCommands
\usepackage{enumitem}
\setlist[description]{style=multiline,leftmargin=7em}
%StopShownPreambleCommands

\begin{document}
\begin{description}
\item[A fairly long label --- so what happens now?]
 Now the label breaks in the margin, but this can still
 look strange if it is long and the item text is short.
\end{description}
\end{document}