Volume increase at the end almost closed bar (4)
I believe the answer is:
veto
'bar' is the definition.
(I've seen this before)
'volume increase at the end almost closed' is the wordplay.
'volume' becomes 'v'.
'at the end' suggests the final letters.
'almost closed' becomes 'to' (I can't explain this - if you can you should give a lot more credence to this answer).
The last letter of 'increase' is 'e'.
'v'+'e'+'to'='VETO'
(Other definitions for veto that I've seen before include "Right to ban" , "Refuse to authorise" , "Throw out" , "A confused vote, altogether negative" , "Right to reject a decision" .)