Tune to be used by church in one of the services (3,5)
I believe the answer is:
air force
'one of the services' is the definition.
(I've seen this before)
'tune to be used by church' is the wordplay.
'tune' becomes 'air' (air can mean a piece of music).
'to' becomes 'for'.
'be used by' says to put letters next to each other.
'church' becomes 'CE' (abbreviation for Church of England).
'air'+'for'+'ce'='AIR FORCE'
'in' is the link.
(Other definitions for air force that I've seen before include "Simple tune for church service" , "Armed service" , "Country's military planes" , "A country's flying service" , "one of the services" .)