In error, since I'm not all-knowing (10)
I believe the answer is:
omniscient
'all-knowing' is the definition.
('omniscient' can be a synonym of 'all-knowing')
'in error since i'm not' is the wordplay.
'in error' indicates anagramming the letters.
'since'+'im'+'not'='sinceimnot'
'sinceimnot' anagrammed gives 'OMNISCIENT'.
(Other definitions for omniscient that I've seen before include "All-knowing" , "Knowing everything" .)