Legally prevent some dishonest opponents (5)
I believe the answer is:
estop
'legally prevent' is the definition.
(I've seen this before)
'some dishonest opponents' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'ESTOP' is hidden in the letters of 'dishonest opponents'.
(Other definitions for estop that I've seen before include "Bar, for a lawyer" , "term in law" , "Preclude certain evidence (at law) - bar (archaic)" , "lawyer's bar" , "Legally hinder removal" .)