A vessel, but overturned (3)
I believe the answer is:
tub
'a vessel' is the definition.
(tub is a kind of vessel)
'but overturned' is the wordplay.
'overturned' says the letters should be written in reverse.
'but' back-to-front is 'TUB'.
(Other definitions for tub that I've seen before include "Wide bucket-like vessel" , "A ranting preacher informally thumps it" , "Pot (of e.g. ice-cream)" , "Large open vessel" , "Unwieldy boat" .)