Clue: Revelation from a policeman must turn up before evidence ultimately plays out (10)
Answer: APOCALYPSE
Explanation: Definition: Revelation
A charade of 'a' plus POC, a revresal ('must turn up' in a down light) of COP ('policeman') plus ALYPS, an anagram ('out') of 'plays' plus E ('evidenc**E** ultimately'), with 'before' indicating the order of the particles.