Growth expert organised Anglicans gathering underground (12)
I believe the answer is:
protuberance
'growth' is the definition.
(protuberance can mean a growth or lump)
'expert organised anglicans gathering underground' is the wordplay.
'expert' becomes 'pro' (short for professional).
'organised' becomes 'ran' (I've seen this before).
'anglicans' becomes 'ce' (Church of England).
'gathering' is an insertion indicator.
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'pro'+'ran'+'ce'='prorance'
'prorance' placed around 'tube' is 'PROTUBERANCE'.
(Other definitions for protuberance that I've seen before include "Something that sticks out" , "Something sticking out" , "Outward bulge - not cupbearer (anag)" , "Swelling" , "Bit jutting out" .)