Revolutionary paramilitary organisation's outfit (4)
I believe the answer is:
sari
'outfit' is the definition.
'revolutionary paramilitary organisation's' is the wordplay.
'revolutionary' shows that the letters should be reversed in order.
'paramilitary organisation's' becomes 'iras' (Irish Republican Army).
'iras' backwards is 'SARI'.
(Other definitions for sari that I've seen before include "Traditional dress of Hindu women" , "habit" , "Traditional Indian women's dress" , "Eastern garment" , "Hindu woman's dress" .)