Bar return of vessels (4)
I believe the answer is:
Here is my best explanation:
'bar' is the definition.
'return of vessels' is the wordplay.
'return of' says the letters should be written in reverse.
'vessels' becomes 'pots' (pot is a kind of vessel).
'pots' in reverse letter order is 'STOP'.
(Other definitions for stop that I've seen before include "colon, say", "Cease; organ voice", "Post (anag.)", "Check", "Prevent - desist", "Refrain from", "Come to an end", "Pack it in - bar", "Don't allow" and "Finish".)