VAT applied to a brass instrument (4)
I believe the answer is:
tuba
'brass instrument' is the definition.
(I know that tuba is a type of brass instrument)
'vat applied to a' is the wordplay.
'vat' becomes 'tub' (synonyms).
'applied to' is a charade indicator (letters next to each other).
'tub'+'a'='TUBA'
(Other definitions for tuba that I've seen before include "horn" , "Deep brass instrument" , "Noisy instrument" , "item band might play" , "part of 14 [ORCHESTRA] ?" .)