Alienate a sergeant, perhaps (8)
I believe the answer is:
estrange
'alienate' is the definition.
(I know that alienate can be written as estrange)
'a sergeant perhaps' is the wordplay.
'perhaps' indicates an anagram.
'sergeant' with letters rearranged gives 'ESTRANGE'.
(Other definitions for estrange that I've seen before include "Get on the wrong side of" , "separate" , "Forfeit friendship of, make hostile" , "Antagonise using reagents" , "Anger set (anag.)" .)