Group assembled for collective action in favour of church (5)
I believe the answer is:
force
'group assembled for collective action' is the definition.
(eg a taskforce)
'in favour of church' is the wordplay.
'in favour of' becomes 'for' (I've seen this before).
'church' becomes 'CE' (abbreviation for Church of England).
'for'+'ce'='FORCE'
(Other definitions for force that I've seen before include "Strength, power" , "Coercion; influence" , "Power - compulsion" , "Strength - compulsion" , "Powerful effect or influence" .)