Compel guard leaving Vatican City church (6)
I believe the answer is:
coerce
'compel' is the definition.
(coercing is a kind of compelling)
'guard leaving vatican city church' is the wordplay.
'guard' becomes 'cover'.
'leaving' suggests deleting specific letters.
'vatican city' becomes 'v'.
'church' becomes 'CE' (abbreviation for Church of England).
'cover' with 'v' taken out is 'coer'.
'coer'+'ce'='COERCE'
(Other definitions for coerce that I've seen before include "Persuade by threatening" , "Forcibly compel" , "Persuade (the unwilling) by force" , "Make or compel" , "Railroad" .)