Religious sect (4)
I believe the answer is:
cult
'religious sect' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cult that I've seen before include "System of religious beliefs and rituals" , "unorthodox organisation" , "Narrow religious sect" , "Unorthodox religion or sect" , "Esoteric religion" .)