Large religious service I have joined (7)
I believe the answer is:
massive
'large' is the definition.
('massive' can be similar in meaning to 'large')
'religious service i have joined' is the wordplay.
'religious service' becomes 'mass' (I've seen this before).
'i have' becomes 'ive'.
'joined' means one lot of letters go next to another.
'mass'+'ive'='MASSIVE'
(Other definitions for massive that I've seen before include "Bulky, weighty" , "Imposing in size" , "Gargantuan" , "Great" , "Not wavering" .)