One to criticise religion (5)
I believe the answer is:
islam
'religion' is the definition.
(Islam is an example)
'one to criticise' is the wordplay.
'one' becomes 'i' (Roman numeral).
'to criticise' becomes 'slam' (I've seen this before**).
'i'+'slam'='ISLAM'
(Other definitions for islam that I've seen before include "Creed" , "Faith with five pillars" , "Monotheistic religion" , "Belief" , "Mails to great eastern religion" .)