Mainly old part of church viewed before storm (2,7)
I believe the answer is:
on average
'mainly' is the definition.
'old part of church viewed before storm' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'part of church' becomes 'nave' ('nave' is part of 'church').
'viewed before' is a charade indicator (letters next to each other).
'storm' becomes 'rage' ('rage' can be a synonym of 'storm').
'o'+'nave'+'rage'='ON AVERAGE'
(Other definitions for on average that I've seen before include "Generally" , "typically speaking" , "Usually" .)