Radical cult ran out of cover (5)
I believe the answer is:
ultra
'radical' is the definition.
(I know that radical can be written as ultra)
'cult ran out of cover' is the wordplay.
'of cover' indicates a hidden word (I've seen 'covering' mean this).
'ULTRA' is hidden in the letters of 'cult ran out'.
(Other definitions for ultra that I've seen before include "Project started at Bletchley Park in 1939 to break the Enigma code" , "Immoderate" , "Extreme (eg in fashion or politics)" , "Political extremist" , "lacking moderation" .)