They say it's the spot for fish! (6)
I believe the answer is:
plaice
'fish' is the definition.
(plaice is a kind of fish)
'they say it's the spot' is the wordplay.
'they say' shows a homophone (sound like).
'it's the spot' becomes 'place' ('place' can be a synonym of 'spot').
'place' sounds like 'PLAICE'.
'for' acts as a link.
(Other definitions for plaice that I've seen before include "Large flat fish" , "Swimmer" , "Flatfish; sounds like put" , "A succulent flat-fish" , "Edible flatfish" .)