Gather there's a church service (5)
I believe the answer is:
amass
'gather' is the definition.
('amass' can be a synonym of 'gather')
'there's a church service' is the wordplay.
'there's' is a charade indicator (letters next to each other).
'church service' becomes 'mass' (I've seen this before).
'a' next to 'mass' is 'AMASS'.
(Other definitions for amass that I've seen before include "Be a compiler" , "Sodium chloride" , "Stockpile" , "Accrue, heap up" , "Collect, hoard" .)