Huge service I have shortened (7)
I believe the answer is:
massive
'huge' is the definition.
(I've seen this in another clue)
'service i have shortened' is the wordplay.
'service' becomes 'mass' (church service).
'i have shortened' becomes 'I've' (contraction of 'I have').
'mass'+'ive'='MASSIVE'
(Other definitions for massive that I've seen before include "Gargantuan" , "Great" , "Enormous" , "Not wavering" , "Exceptionally large" .)