Doubtful one acting so strangely (8)
I believe the answer is:
agnostic
'doubtful' is the definition.
(thesaurus)
'one acting so strangely' is the wordplay.
'strangely' is an anagram indicator.
'acting'+'so'='actingso'
'actingso' with letters rearranged gives 'AGNOSTIC'.
(Other definitions for agnostic that I've seen before include "uncertain about what to believe" , "One who is undecided" , "Coasting as one who believes the mind cannot know anything of a god" , "Doubting, non-commital" , "One believing we can know nothing of the existence of any god" .)