Deliberately set fire to church by rocky peak (5)
I believe the answer is:
torch
'deliberately set fire to' is the definition.
(I've seen this before)
'church by rocky peak' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'by' means one lot of letters go next to another.
'rocky peak' becomes 'tor' (I've seen this before).
'ch' after 'tor' is 'TORCH'.
(Other definitions for torch that I've seen before include "source of enlightenment" , "Source of light" , "Such a love song" , "may get one glowing" , "A large candle" .)