Priest is out to stop make-believe (7)
I believe the answer is:
pretend
'make-believe' is the definition.
('pretend' can be a synonym of 'make-believe')
'priest is out to stop' is the wordplay.
'out' is a deletion indicator.
'to stop' becomes 'end' ('end' can be a synonym of 'stop'**).
'priest' with 'is' taken away is 'pret'.
'pret'+'end'='PRETEND'
(Other definitions for pretend that I've seen before include "Imaginary, make-believe" , "Dissemble" , "Simulated" , "bluff" , "Imagined" .)