Northern priest engaged by big story as source of humour (3-5)
I believe the answer is:
one-liner
'source of humour' is the definition.
(a one-line joke)
'northern priest engaged by big story' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'priest' becomes 'eli' (biblical figure).
'engaged by' indicates putting letters inside (I've seen this in other clues).
'big story' becomes 'oner' (term for a remarkable story).
'n'+'eli'='neli'
'neli' put into 'oner' is 'ONE-LINER'.
'as' is the link.
(Other definitions for one-liner that I've seen before include "Pithy joke" , "Quick joke?" , "Witty remark contained in a single sentence" , "Short joke - witty remark" , "Witty remark consisting of a short sentence" .)