Gather for a religious service (5)
I believe the answer is:
amass
'gather' is the definition.
(amass can mean to collect or gather)
'a religious service' is the wordplay.
'religious service' becomes 'mass' (mass in a church).
'a'+'mass'='AMASS'
'for' is the link.
(Other definitions for amass that I've seen before include "Put together" , "Stock up on" , "Gather in great quantity" , "Assemble" , "Accumulate over time" .)