Extremist leaderless sect attracting artist (5)
I believe the answer is:
ultra
'extremist' is the definition.
(ultra can mean an extremist or radical)
'leaderless sect attracting artist' is the wordplay.
'leaderless' means to remove the first letter.
'sect' becomes 'cult' (I've seen this before).
'attracting' means one lot of letters go next to another.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'cult' with its first letter removed is 'ult'.
'ult'+'ra'='ULTRA'
(Other definitions for ultra that I've seen before include "such sound treatment?" , "lacking moderation" , "Prefix meaning extreme or excessive" , "(One who is) extreme (in eg political opinion)" , "Prefix meaning excessively or extremely" .)