I'm no sceptic, troubled about delusion (13)
I believe the answer is:
misconception
'delusion' is the definition.
(thesaurus)
'i'm no sceptic troubled about' is the wordplay.
'troubled' is an anagram indicator.
'about' becomes 'on' (**).
'im'+'no'+'sceptic'='imnosceptic'
'imnosceptic' is an anagram of 'misconcepti'.
'misconcepti'+'on'='MISCONCEPTION'
(Other definitions for misconception that I've seen before include "Delusion - wrong idea" , "Wrong view or opinion" , "Mistake" , "False view - economic pints (anag)" .)