Unorthodox sect (4)
I believe the answer is:
cult
'unorthodox 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 or false religion" , "unorthodox organisation" , "Group with system of formal worship" , "Narrow religious sect" .)