Prevent vessels keeling over (4)
I believe the answer is:
stop
'prevent' is the definition.
(stopping is a kind of preventing)
'vessels keeling over' is the wordplay.
'vessels' becomes 'pots' (pot is a kind of vessel).
'keeling over' shows that the letters should be reversed in order.
'pots' reversed gives 'STOP'.
(Other definitions for stop that I've seen before include "Hold on" , "Halt; finish" , "Pause - prevent" , "cut out" , "Put an end to" .)