Deep sea fish (4)
I believe the answer is:
bass
I believe this clue is a double definition.
'deep' is the first definition.
('bass' can be a synonym of 'deep')
'sea fish' is the second definition.
(bass is a kind of sea fish)
(Other definitions for bass that I've seen before include "Sea perch - male voice" , "Fish for the low-voiced" , "Large fish - singer" , "bottom of sound?" , "Low singing voice" .)