Set fire to church by rocky peak (5)
I believe the answer is:
torch
'set fire to' is the definition.
(I've seen this before)
'church by rocky peak' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'by' says to put letters next to each other.
'rocky peak' becomes 'tor' (a tor is a rocky hill).
'ch' after 'tor' is 'TORCH'.
(Other definitions for torch that I've seen before include "Hand-held lamp or burning stick" , "Deliberately set fire to" , "Link" , "A large candle" , "source of illumination" .)