Brute causes a disturbance internally (6)
I believe the answer is:
sadist
'brute' is the definition.
'causes a disturbance internally' is the wordplay.
'internally' indicates the answer is hidden within the clue.
'SADIST' is hidden within 'causes a disturbance'.
(Other definitions for sadist that I've seen before include "An amalgam, made up of two or more parts" , "One who loves being cruel" , "One who takes pleasure in cruelty" , "Cruel type" , "One taking pleasure in cruelty" .)