Fish deep (4)
I believe the answer is:
bass
I believe this clue is a double definition.
'fish' is the first definition.
(bass is a kind of fish)
'deep' is the second definition.
(synonyms)
(Other definitions for bass that I've seen before include "Fish; deep voice" , "A fish, or the lowest pitch in music" , "Variety of sea and freshwater perch" , "Lowest adult male singing voice" , "bottom of sound?" .)