A million visiting church for summit (4)
I believe the answer is:
acme
'summit' is the definition.
(synonyms)
'a million visiting church' is the wordplay.
'million' becomes 'm' (abbreviation).
'visiting' is an insertion indicator.
'church' becomes 'CE' (abbreviation for Church of England).
'm' put inside 'ce' is 'cme'.
'a'+'cme'='ACME'
'for' is the link.
(Other definitions for acme that I've seen before include "Point of perfection, apex" , "highest position" , "Point of maximum development" , "Highest point of excellence or achievement" , "Zenith - optimum" .)