Warning light at the back of church room (7)
I believe the answer is:
chamber
'room' is the definition.
(I know that room can be written as chamber)
'warning light at the back of church' is the wordplay.
'warning light' becomes 'amber' (I've seen this before).
'at the back of' says to put letters next to each other.
'church' becomes 'ch' (common abbreviation).
'amber' put after 'ch' is 'CHAMBER'.
(Other definitions for chamber that I've seen before include "Enclosed cavity, part of gun" , "Bedroom" , "posh room" , "Room for the judge or the bed" , "Compartment" .)