Good man to encourage fish (8)
I believe the answer is:
sturgeon
'fish' is the definition.
(sturgeon is a kind of fish)
'good man to encourage' is the wordplay.
'good man' becomes 'st'.
'to encourage' becomes 'urgeon' (urging on is a kind of encouraging).
'st'+'urgeon'='STURGEON'
(Other definitions for sturgeon that I've seen before include "Fish - got nurse (anag)" , "swimmer" , "Fish producing caviare" , "caviar suppliers" , "producer of caviar" .)