Alienate troublesome sergeant (8)
I believe the answer is:
estrange
'alienate' is the definition.
(I know that alienate can be written as estrange)
'troublesome sergeant' is the wordplay.
'troublesome' indicates an anagram.
'sergeant' anagrammed gives 'ESTRANGE'.
(Other definitions for estrange that I've seen before include "Antagonise using reagents" , "Cause to be alienated" , "As Regent (anag.)" , "Anger set (anag.)" , "Become unfriendly towards sergeant" .)