Aria, say, very much for bass? Not quite (4)
I believe the answer is:
solo
'aria' is the definition.
(thesaurus)
'very much for bass? not quite' is the wordplay.
'very much' becomes 'so'.
'for' means one lot of letters go next to another.
'bass?' becomes 'low' ('low' can be similar in meaning to 'bass').
'not quite' means to remove the last letter.
'low' with its final letter taken away is 'lo'.
'so'+'lo'='SOLO'
'say' acts as a link.
(Other definitions for solo that I've seen before include "Performance by one person" , "Unaccompanied passage" , "Lone" , "One-person performance" , "Piece of music for one" .)