Extremist contributing to awful tragedy (5)
I believe the answer is:
ultra
'extremist' is the definition.
('ultra' can be a synonym of 'extremist')
'contributing to awful tragedy' is the wordplay.
'contributing to' indicates a hidden word.
'ULTRA' is hidden in the letters of 'awful tragedy'.
(Other definitions for ultra that I've seen before include "such sound treatment?" , "Secret intelligence" , "Extreme - extremist" , "Project started at Bletchley Park in 1939 to break the Enigma code" , "Political extremist" .)