Person acting so dubiously? (8)
I believe the answer is:
agnostic
'person' is the definition.
(agnostic is a kind of person)
'acting so dubiously?' is the wordplay.
'dubiously?' indicates anagramming the letters (I've seen 'dubiously' mean this).
'acting'+'so'='actingso'
'actingso' anagrammed gives 'AGNOSTIC'.
(Other definitions for agnostic that I've seen before include "He finds it difficult to believe" , "Religiously non-commital" , "Doubtful, non-committal" , "Coasting (anag)" , "Incredulous person" .)