Sea bass (4)
I believe the answer is:
deep
I believe this clue is a double definition.
'sea' is the first definition.
(thesaurus)
'bass' is the second definition.
(I know that bass can be written as deep)
(Other definitions for deep that I've seen before include "Sunk low" , "Profound - cunning - sea - bass" , "hard to understand" , "Intense; low (in pitch)" , "Wise - difficult to understand" .)