Priest once more dismissing one heathen (5)
I believe the answer is:
pagan
'heathen' is the definition.
(both can mean an irreligious person)
'priest once more dismissing one' is the wordplay.
'priest' becomes 'p' (ecclesiastical abbreviation).
'once more' becomes 'again' (synonyms).
'dismissing' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'again' with 'i' taken away is 'agan'.
'p'+'agan'='PAGAN'
(Other definitions for pagan that I've seen before include "Person without any conventional religion" , "Polytheist" , "A heathen, not a Christian" , "One not accepting your god" , "Not conventionally religious" .)