Plot against part of church (8)
I believe the answer is:
conspire
'plot' is the definition.
(conspiring is a kind of plotting)
'against part of church' is the wordplay.
'against' becomes 'con' (I've seen this before).
'part of church' becomes 'spire'.
'con'+'spire'='CONSPIRE'
(Other definitions for conspire that I've seen before include "scheme" , "'Plot, connive (8)'" , "Plot together - on prices (anag)" , "Act secretly with others" , "Secretly plan together" .)