No rating could be so unenlightened! (8)
I believe the answer is:
ignorant
'unenlightened' is the definition.
('ignorant' can be a synonym of 'unenlightened')
'no rating could be' is the wordplay.
'could be' is an anagram indicator.
'no'+'rating'='norating'
'norating' anagrammed gives 'IGNORANT'.
'so' is the link.
(Other definitions for ignorant that I've seen before include "Lacking knowledge or awareness" , "Ill-educated" , "Lacking knowledge or lacking manners" , "Crass" , "Unaware, uninformed" .)