"Set for fish," you say (5)
I believe the answer is:
place
'set' is the definition.
(synonyms)
'fish you say' is the wordplay.
'fish' becomes 'plaice' (plaice is a kind of fish).
'you say' shows a homophone (sound like).
'plaice' is a homophone of 'PLACE'.
'for' acts as a link.
(Other definitions for place that I've seen before include "Locality" , "Endearingly pretty" , "Venue" , "Locate" , "Locus" .)