Deep fish (4)
I believe the answer is:
bass
This clue is a double definition.
'deep' is the first definition.
(synonyms)
'fish' is the second definition.
(bass is a kind of fish)
(Other definitions for bass that I've seen before include "Deep-toned" , "A fish, or the lowest pitch in music" , "Fish (which sings?)" , "Scottish rock" , "Fish; male singing voice" .)