VAT on a musical instrument (4)
I believe the answer is:
tuba
'musical instrument' is the definition.
(tuba is a kind of musical instrument)
'vat on a' is the wordplay.
'vat' becomes 'tub' (synonyms).
'on' is a charade indicator (letters next to each other).
'tub'+'a'='TUBA'
(Other definitions for tuba that I've seen before include "A bass instrument" , "Producer of note" , "member of band" , "bit of brass" , "It's not top brass" .)