Permit parasites to join new church (7)
I believe the answer is:
licence
'permit' is the definition.
(I know that licence is a more specific form of the action authorise)
'parasites to join new church' is the wordplay.
'parasites' becomes 'lice' (lice are parasitic insects).
'to join' is a charade indicator (letters next to each other).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'church' becomes 'CE' (abbreviation for Church of England).
'lice'+'n'+'ce'='LICENCE'
(Other definitions for licence that I've seen before include "Permit - excessive behaviour" , "See 1" , "Freedom of action" , "Leave" , "Formal permit" .)