A head of state entering church in a suit (4)
I believe the answer is:
case
'a suit' is the definition.
('case' can be a synonym of 'suit')
'a head of state entering church' is the wordplay.
'head of' says to take the initial letters.
'entering' is an insertion indicator.
'church' becomes 'ce' (abbreviation for Church of England**).
The initial letter of 'state' is 's'.
'a'+'s'='as'
'as' put into 'ce' is 'CASE'.
'in' is the link.
(Other definitions for case that I've seen before include "Wine buyer's basic quantity" , "proceedings" , "Keep careful watch on" , "A law suit or a container" , "Piece of luggage and a lawyer's proceeding" .)