Occurrence archdeacon observed in sci-fi film (5)
I believe the answer is:
event
'occurrence' is the definition.
(thesaurus)
'archdeacon observed in sci-fi film' is the wordplay.
'archdeacon' becomes 'ven'.
'observed in' is an insertion indicator.
'sci-fi film' becomes 'et' (I've seen this in other clues).
'ven' inserted within 'et' is 'EVENT'.
(Other definitions for event that I've seen before include "contest in programme" , "Anything which happens" , "Notable occasion" , "Occurence, happening" , "A notable occurrence or happening" .)