\starttext

\unexpanded\def\ShowSummary#1#2#3%
  {\inleftmargin{#1}\structurelistuservariable{summary}\blank}


\placelist[chapter][alternative=command,command=\ShowSummary]

\startchapter
  [title={This is a chapter}]
  [summary={This is a small description of this chapter}]

\input knuth.tex

\stopchapter

\startchapter
  [title={This is also a chapter}]
  [summary={This is a larger description of this chapter}]

\input ward.tex

\stopchapter

\stoptext
