Fish of the deep (4)
I believe the answer is:
Here is my best explanation:
I believe this is a double definition.
'fish' is the first definition.
(I've seen this before)
'the deep' is the second definition.
'of' is the link.
(Other definitions for bass that I've seen before include "Deep-toned", "Variety of sea and freshwater perch", "A fish; a voice", "Fish for the low-voiced", "Sea perch - male voice", "Fish - singer", "Male voice - fish", "Type of fish", "bottom of sound?" and "deep in the sound".)