Spot, so we hear, for fish (6)
I believe the answer is:
plaice
'for fish' is the definition.
(plaice is a kind of fish)
'spot so we hear' is the wordplay.
'spot' becomes 'place' (synonyms).
'so we hear' shows a homophone (sound like).
'place' sounds like 'PLAICE'.
(Other definitions for plaice that I've seen before include "N. Atlantic flatfish" , "Swimmer" , "Large flatfish" , "A succulent flat-fish" , "Position the fish, we hear" .)