Gather church service is led by archbishop principally (5)
I believe the answer is:
amass
'gather' is the definition.
('amass' can be a synonym of 'gather')
'church service is led by archbishop principally' is the wordplay.
'church service' becomes 'mass' (I've seen this before).
'is led by' means one lot of letters go next to another.
'principally' says to take the initial letters.
The initial letter of 'archbishop' is 'a'.
'mass' after 'a' is 'AMASS'.
(Other definitions for amass that I've seen before include "Sodium chloride" , "Accumulate over time" , "Stockpile" , "store" , "Accumulate, pile up" .)