Fish of the deep (4)
I believe the answer is:
bass
I believe this is a double definition.
'fish' is the first definition.
(I've seen this before)
'the deep' is the second definition.
(thesaurus)
'of' is the link.
(Other definitions for bass that I've seen before include "Low fellow" , "Performer" , "Sea perch - male voice" , "part of score" , "Fish, guitar" .)