Cult rapper, somewhat extreme (5)
I believe the answer is:
ultra
'extreme' is the definition.
(I've seen this before)
'cult rapper somewhat' is the wordplay.
'somewhat' says the answer is hidden in the clue.
'ULTRA' is hidden within 'cult rapper'.
(Other definitions for ultra that I've seen before include "Over the top" , "Political extremist" , "(One who is) extreme (in eg political opinion)" , "lacking moderation" , "Saharan dweller" .)