Mainly the church at Filey, possibly (7)
I believe the answer is:
chiefly
'mainly' is the definition.
(I know that mainly can be written as chiefly)
'church at filey possibly' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'at' means one lot of letters go next to another.
'possibly' is an anagram indicator.
'filey' is an anagram of 'iefly'.
'ch'+'iefly'='CHIEFLY'
'the' acts as a link.
(Other definitions for chiefly that I've seen before include "In the main" , "Mainly, principally" , "For the most part" .)