Priest always in action must be paid (8)
I believe the answer is:
defrayed
'paid' is the definition.
(defraying is a kind of paying)
'priest always in action' is the wordplay.
'priest' becomes 'fr'.
'always' becomes 'ay'.
'in' is an insertion indicator.
'action' becomes 'deed' (deed is a kind of action).
'fr'+'ay'='fray'
'fray' going inside 'deed' is 'DEFRAYED'.
'must be' is the link.
(Other definitions for defrayed that I've seen before include "Put paid to" , "Paid, settled (a cost)" .)