Yon church’s roof (6)
I believe the answer is:
thatch
'roof' is the definition.
(thatch is a kind of roof)
'yon church's' is the wordplay.
'yon' becomes 'that' (I've seen this before).
'church' becomes 'ch' (common abbreviation).
'that'+'ch'='THATCH'
(Other definitions for thatch that I've seen before include "An old-fashioned roof covering of straw, reeds etc" , "Old-fashioned roofing material" , "Traditional material for covering" , "Organic material for old rustic roof" , "nothing of interest to thief wanting roof lead" .)