Gather together for a church service (5)
I believe the answer is:
amass
'gather together' is the definition.
'a church service' is the wordplay.
'church service' becomes 'mass' (I've seen this before).
'a'+'mass'='AMASS'
'for' is the link.
(Other definitions for amass that I've seen before include "Stockpile" , "Sodium chloride" , "Accrue, heap up" , "Collect, hoard" , "Accumulate over time" .)