Acting so uncertain — is one? (8)
I believe the answer is:
agnostic
'one?' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?
'acting so uncertain' is the wordplay.
'uncertain' is an anagram indicator.
'acting'+'so'='actingso'
'actingso' is an anagram of 'AGNOSTIC'.
'is' acts as a link.
(Other definitions for agnostic that I've seen before include "someone like Pyrrho" , "Incredulous person" , "One doubts" , "Sceptic - unbelieving" , "Unbeliever; sceptic" .)