VAT only goes up (3)
I believe the answer is:
tub
'vat' is the definition.
(I've seen this before)
'only goes up' is the wordplay.
'only' becomes 'but' (I've seen this before).
'goes up' says the letters should be written in reverse.
'but' written backwards gives 'TUB'.
(Other definitions for tub that I've seen before include "Small cask" , "Barrel; boat" , "Unwieldy boat" , "Carton" , "An informal bath or boat" .)