Sea-bass (4)
I believe the answer is:
deep
I believe this is a double definition.
'sea' is the first definition.
(thesaurus)
'bass' is the second definition.
('deep' can be a synonym of 'bass')
(Other definitions for deep that I've seen before include "Heartfelt" , "profoundly" , "Extending far down" , "hard to understand" , "Bottomless" .)