Somewhat difficult, radical fanatic (5)
I believe the answer is:
ultra
'fanatic' is the definition.
(I've seen this before)
'somewhat difficult radical' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'ULTRA' can be found hidden inside 'difficult radical'.
(Other definitions for ultra that I've seen before include "Political extremist" , "(As prefix) beyond or extreme" , "Over the top" , "Fanatic" , "Prefix meaning beyond or excessively" .)