Sceptic acting so strangely (8)
I believe the answer is:
agnostic
'sceptic' is the definition.
(I've seen this before)
'acting so strangely' is the wordplay.
'strangely' indicates an anagram.
'acting'+'so'='actingso'
'actingso' anagrammed gives 'AGNOSTIC'.
(Other definitions for agnostic that I've seen before include "doubtful, I think" , "I doubt" , "A non-believer in God, coasting along" , "Song I act (anag)" , "Coasting as one who believes the mind cannot know anything of a god" .)