Sceptic may be 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'.
'may be' is the link.
(Other definitions for agnostic that I've seen before include "One who is undecided" , "Sceptic - unbelieving" , "Unbeliever; sceptic" , "Song I act (anag)" , "Doubtful, non-committal" .)