One slice each (6)
I believe the answer is:
apiece
'each' is the definition.
(synonyms)
'one slice' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'slice' becomes 'piece' (synonyms).
'a'+'piece'='APIECE'
(Other definitions for apiece that I've seen before include "For or by each one of group" , "Per person" , "for everyone" , "A head" .)