Each pastrycook's product is in one (6)
I believe the answer is:
apiece
'each' is the definition.
(synonyms)
'pastrycook's product is in one' is the wordplay.
'pastrycook's product' becomes 'pie' (I can't justify this - if you can you should believe this answer much more).
'is in' is an insertion indicator.
'one' becomes 'ace' (ace has a value of one in some card games).
'pie' inserted within 'ace' is 'APIECE'.
Can you help me to learn more?
(Other definitions for apiece that I've seen before include "A head" , "for everyone" , "For or by each one of group" , "Per person" .)