Eating pastry, one each (6)
I believe the answer is:
apiece
'each' is the definition.
(synonyms)
'eating pastry one' is the wordplay.
'eating' indicates putting letters inside.
'pastry' becomes 'pie' (pie is a kind of pastry).
'one' becomes 'ace' (synonyms).
'pie' inserted into 'ace' is 'APIECE'.
(Other definitions for apiece that I've seen before include "A head" , "for everyone" , "For or by each one of group" , "Per person" .)