Prevent, in law (5)
I believe the answer is:
estop
'prevent in law' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for estop that I've seen before include "Legal term" , "Hinder, preclude (law)" , "lawyer's bar" , "Bar, for a lawyer" , "Preclude certain evidence (at law) - bar (archaic)" .)