Mother joins the saints in a church service (4)
I believe the answer is:
mass
'a church service' is the definition.
(Mass is a type of church service)
'mother joins the saints' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'joins' says to put letters next to each other.
'the saints' becomes 'ss' (an abbreviation for saint written twice).
'ma'+'ss'='MASS'
'in' is the link.
(Other definitions for mass that I've seen before include "state, in short" , "Lump of material" , "Rite; quantity of matter" , "RC ceremony" , "Lots of people" .)