Gather there's a service (5)
I believe the answer is:
amass
'gather' is the definition.
('amass' can be a synonym of 'gather')
'there's a service' is the wordplay.
'there's' means one lot of letters go next to another.
'service' becomes 'mass' (I've seen this in other clues).
'a'+'mass' is 'AMASS'.
(Other definitions for amass that I've seen before include "Stockpile" , "Gather together, pile up" , "Gather in great quantity" , "Build up" , "Collect or gather" .)