A sceptic acting so strangely (8)
I believe the answer is:
agnostic
'a 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" , "One believes that nothing can be known about the existence of God" , "Religiously non-commital" , "Questioning" , "Holding that knowledge of Supreme Being is impossible" .)