Church includes demolished prison plot (8)
I believe the answer is:
'plot' is the definition.
(conspiring is a kind of plotting)
'church includes demolished prison' is the wordplay.
'church' becomes 'CE' (abbreviation for Church of England).
'includes' means one lot of letters goes inside another.
'demolished' indicates an anagram.
'prison' with letters rearranged gives 'onspir'.
'ce' going around 'onspir' is 'CONSPIRE'.
(Other definitions for conspire that I've seen before include "scheme" , "So prince is given to plan secretly with others" , "Act secretly with others" , "Collude" , "Secretly plan together" .)