Extremist’s quality real, but regularly overlooked (5)
I believe the answer is:
ultra
'extremist's' is the definition.
(ultra can mean an extremist or radical)
'quality real but regularly overlooked' is the wordplay.
'quality' becomes 'ult' (I can't explain this - if you can you should believe this answer much more).
'but regularly overlooked' indicates alternate letters (letters removed at regular intervals).
The alternating letters of 'real' are 'ra'.
'ult'+'ra'='ULTRA'
Can you help me to learn more?
(Other definitions for ultra that I've seen before include "Secret intelligence" , "Political extremist" , "extraordinary" , "extreme measures" , "Above, beyond, super" .)