Cad, revolting man, leading church? Look out now! (8)
I believe the answer is:
dogwatch
'look out now' is the definition.
(a watch carried out on a ship)
'cad revolting man leading church?' is the wordplay.
'cad' becomes 'dog' (synonyms).
'revolting man' becomes 'wat' (Wat Tyler, leader of the Peasants' Revolt).
'leading' says to put letters next to each other.
'church?' becomes 'ch' (common abbreviation).
'dog'+'wat'+'ch'='DOGWATCH'
(Other definitions for dogwatch that I've seen before include "short spell on board" , "Short afternoon shift on ship" , "4-6pm or 6-8pm shift on a ship" , "period of vigil" , "Duty period for ship's complement, kind of canine alert" .)