Philistine stripped and thrashed sceptic (8)
I believe the answer is:
nihilist
'sceptic' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
I cannot understand how the rest of the clue works.
(Other definitions for nihilist that I've seen before include "Revolutionary" , "Supporter of 15 [NIX]" , "One rejecting authority, values" , "One who believes in nothing at all" , "Person who rejects all moral and religious principles - nothing really exists" .)