Sceptic acting so oddly (8)
I believe the answer is:
agnostic
'sceptic' is the definition.
(I've seen this before)
'acting so oddly' is the wordplay.
'oddly' is an anagram indicator.
'acting'+'so'='actingso'
'actingso' is an anagram of 'AGNOSTIC'.
(Other definitions for agnostic that I've seen before include "Holding that knowledge of Supreme Being is impossible" , "One believing God is unknowable" , "Not sure" , "One believing we can know nothing of the existence of any god" , "He finds it difficult to believe" .)