Extremist starts to unleash lethal terror, rage, aggression (5)
I believe the answer is:
ultra
'extremist' is the definition.
('ultra' can be a synonym of 'extremist')
'starts to unleash lethal terror rage aggression' is the wordplay.
'starts to' indicates taking the first letters.
The first letters of 'unleash lethal terror rage aggression' is 'ULTRA'.
(Other definitions for ultra that I've seen before include "Violet's predecessor" , "Extreme (eg in fashion or politics)" , "Project started at Bletchley Park in 1939 to break the Enigma code" , "Far beyond the norm" , "Fanatic" .)