In my experience, you need to "glue" the parts together with a macro, which also means you need to use a macrocall widget for invocation.
Using the new "robust" synatax with "__", it might look like:
\define deref(tid fld) <$transclude tiddler=<<__tid__>> field=<<__fld__>> />
<$macrocall $name="deref" tid={{!!summary}} fld="caption"/>
HTH
-- Mark