Leader ousted from fringe organisation (5)
I believe the answer is:
order
'organisation' is the definition.
(I know that order is a type of organisation)
'leader ousted from fringe' is the wordplay.
'leader ousted from' means to remove the first letter.
'fringe' becomes 'border' (fringe can mean a border or edge).
'border' with its initial letter taken off is 'ORDER'.
(Other definitions for order that I've seen before include "rule of law" , "Class - instruction - book" , "architectural style" , "pattern" , "An authoritative command" .)