Each master eats pastry (6)
I believe the answer is:
apiece
'each' is the definition.
(synonyms)
'master eats pastry' is the wordplay.
'master' becomes 'ace' (I've seen this in another clue).
'eats' means one lot of letters goes inside another.
'pastry' becomes 'pie' (pie is a kind of pastry).
'ace' enclosing 'pie' is 'APIECE'.
(Other definitions for apiece that I've seen before include "A head" , "For or by each one of group" , "for everyone" , "Per person" .)