Prohibit, legally (5)
I believe the answer is:
estop
'prohibit legally' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for estop that I've seen before include "Legal term" , "Legally hinder removal" , "Preclude certain evidence (at law) - bar (archaic)" , "Legally bar" , "Legally check" .)