Good! Fish, fish, fish! (8)
I believe the answer is:
grayling
'fish' is the definition.
(grayling is a type of fish)
'good fish fish' is the wordplay.
'good' becomes 'g' (abbreviation).
'fish' becomes 'ray' (ray is a kind of fish).
'fish' becomes 'ling' (ling is a kind of fish).
'g'+'ray'+'ling'='GRAYLING'
(Other definitions for grayling that I've seen before include "Butterfly" , "Salmonoid fish" , "Fish of the salmon family" , "Catch" , "Fish - philosopher" .)