Gather it's a church service (5)
I believe the answer is:
amass
'gather' is the definition.
(amass can mean to collect or gather)
'a church service' is the wordplay.
'church service' becomes 'mass' (Mass is a type of church service).
'a'+'mass'='AMASS'
'it's' acts as a link.
(Other definitions for amass that I've seen before include "Gather in great quantity" , "Gathers" , "Accumulate, heap up" , "Put together" , "Build up" .)