Sceptic acting so badly (8)
I believe the answer is:
agnostic
'sceptic' is the definition.
(I've seen this before)
'acting so badly' is the wordplay.
'badly' indicates an anagram.
'acting'+'so'='actingso'
'actingso' is an anagram of 'AGNOSTIC'.
(Other definitions for agnostic that I've seen before include "One doubts" , "One believing God is unknowable" , "Sceptic - unbelieving" , "Doubter - coatings (anag)" , "Religiously non-commital" .)