Belief one’s got to escape (5)
I believe the answer is:
islam
'belief' is the definition.
(I've seen this before)
'one's got to escape' is the wordplay.
'one's' becomes 'i's' (Roman numeral).
'got' is a charade indicator (letters next to each other).
'to escape' becomes 'lam' (lam is a kind of escape).
'is'+'lam'='ISLAM'
(Other definitions for islam that I've seen before include "Monotheistic religion" , "organised religion" , "Religion of the Koran" , "Belief" , "Religion based on teachings of Muhammad" .)