Deliberately set fire to roof of temple or church (5)
I believe the answer is:
torch
'deliberately set fire to' is the definition.
(I've seen this before)
'roof of temple or church' is the wordplay.
'roof of' indicates taking the first letters (in a down clue, the top letter).
'church' becomes 'ch' (common abbreviation).
The initial letter of 'temple' is 't'.
't'+'or'+'ch'='TORCH'
(Other definitions for torch that I've seen before include "Hand-held lamp or burning stick" , "Portable lamp; set alight" , "Portable light; burn" , "Link" , "Set fire to - hand-held light" .)