Main Church holding new spiritualist meeting (6)
I believe the answer is:
seance
'spiritualist meeting' is the definition.
(I've seen this before)
'main church holding new' is the wordplay.
'main' becomes 'sea' (the main is an old name for the sea).
'church' becomes 'CE' (abbreviation for Church of England).
'holding' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'sea'+'ce'='seace'
'seace' placed around 'n' is 'SEANCE'.
(Other definitions for seance that I've seen before include "Session for spiritualists" , "Spiritualist sitting" , "Meeting with spiritualist present" , "Spiritualistic session" , "Sitting with a medium" .)