Mother joins the saints in a church service (4)
I believe the answer is:
mass
'a church service' is the definition.
(thesaurus)
'mother joins the saints' is the wordplay.
'mother' becomes 'ma' (ma is a kind of mother).
'joins' says to put letters next to each other.
'the saints' becomes 'ss'.
'ma'+'ss'='MASS'
'in' acts as a 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" .)