Person acting so dubious? (8)
I believe the answer is:
agnostic
'person' is the definition.
(agnostic is a kind of person)
'acting so dubious?' is the wordplay.
'dubious?' indicates anagramming the letters (I've seen 'dubious' mean this).
'acting'+'so'='actingso'
'actingso' anagrammed gives 'AGNOSTIC'.
(Other definitions for agnostic that I've seen before include "One believes that nothing can be known about the existence of God" , "Incredulous person" , "Unbeliever; sceptic" , "Coasting with unbeliever" , "someone like Pyrrho" .)