Church on rocky hill set ablaze (5)
I believe the answer is:
torch
'set ablaze' is the definition.
(I've seen this in another clue)
'church on rocky hill' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'on' says to put letters next to each other.
'rocky hill' becomes 'tor' (I've seen this before).
'ch' after 'tor' is 'TORCH'.
(Other definitions for torch that I've seen before include "source of illumination" , "Source of light" , "Set fire to (something)" , "Such a love song" , "Set alight deliberately" .)