Prevent vessels returning (4)
I believe the answer is:
stop
'prevent' is the definition.
(stopping is a kind of preventing)
'vessels returning' is the wordplay.
'vessels' becomes 'pots' (pot is a kind of vessel).
'returning' shows that the letters should be reversed in order.
'pots' back-to-front is 'STOP'.
(Other definitions for stop that I've seen before include "Cease, halt" , "Pack it in - bar" , "Intercept" , "Part of organ" , "Plug; cease" .)