Church season less defiled after dismissing priest, an opportunist (10)
I believe the answer is:
adventurer
'opportunist' is the definition.
(I have seen 'An opportunist' mean 'adventurer' so perhaps 'opportunist' could also mean 'adventurer')
'church season less defiled after dismissing priest an' is the wordplay.
'church season' becomes 'advent' (I've seen this before).
'less defiled' becomes 'ur' (I am not sure about this - if you are sure you should believe this answer much more).
'after dismissing' is a deletion indicator.
'priest' becomes 'p' (ecclesiastical abbreviation).
'an' becomes 'per' (synonymous as in 'once an hour', 'once per hour').
'per' with 'p' taken away is 'er'.
'advent'+'ur'+'er'='ADVENTURER'
Can you help me to learn more?
(Other definitions for adventurer that I've seen before include "Daring traveller" , "Drake perhaps" , "hero?" , "one to take risks" , "An opportunist" .)