Oppose extremists in tough conflict close to militant (6)
I believe the answer is:
thwart
'oppose' is the definition.
(thesaurus)
'extremists in tough conflict close to militant' is the wordplay.
'extremists in' means to remove the middle letters.
'conflict' becomes 'war' (war is a kind of conflict).
'close to' says to take the final letters (the close/ending of).
'tough' with its centre taken out is 'th'.
The final letter of 'militant' is 't'.
'th'+'war'+'t'='THWART'
(Other definitions for thwart that I've seen before include "seat for oarsman" , "Frustrate, prevent from doing" , "Defeat" , "Prevent from accomplishing something" , "Oppose successfully" .)