Fish of the deep (4)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'fish' is the first definition.
(bass is a kind of fish)
'the deep' is the second definition.
'of' is the link.
(Other definitions for bass that I've seen before include "Singing voice - fish", "Fish (which sings?)", "A fish, or the lowest pitch in music", "Singer; fish", "Male singer and saltwater fish", "Lowest male singer", "Low singing voice", "Lowest adult male singing voice", "Low-pitched; a fish" and "Variety of sea and freshwater perch".)