Undermine rebellious brutes, grabbing victory (7)
I believe the answer is:
subvert
'undermine' is the definition.
('subvert' can be a synonym of 'undermine')
'rebellious brutes grabbing victory' is the wordplay.
'rebellious' is an anagram indicator.
'grabbing' indicates putting letters inside.
'victory' becomes 'v'.
'brutes' anagrammed gives 'subert'.
'subert' going around 'v' is 'SUBVERT'.
(Other definitions for subvert that I've seen before include "Cause the downfall of; undermine" , "Overthrow - corrupt" , "Sabotage" , "Undermine (someone's authority)" , "upset" .)