Spiny-finned fish (4)
I believe the answer is:
bass
'spiny-finned fish' is the definition.
(bass is a kind of spiny-finned fish)
This is the entire clue.
(Other definitions for bass that I've seen before include "Fish; male singing voice" , "Large percussion instrument" , "Fish (which sings?)" , "Low-pitched; a fish" , "bottom of sound?" .)