Legally prevent Eastern vessels returning (5)
I believe the answer is:
estop
'legally prevent' is the definition.
(I've seen this in another clue)
'eastern vessels returning' is the wordplay.
'eastern' becomes 'e'.
'vessels' becomes 'pots' (pot is a kind of vessel).
'returning' says the letters should be written in reverse.
'pots' back-to-front is 'stop'.
'e'+'stop'='ESTOP'
(Other definitions for estop that I've seen before include "lawyer's bar" , "Legally prevent" , "Legally check" , "Bar, for a lawyer" , "Hinder, preclude (law)" .)