One knight celebrated but notorious (8)
I believe the answer is:
infamous
'notorious' is the definition.
(synonyms)
'one knight celebrated' is the wordplay.
'one' becomes 'i' (Roman numeral).
'knight' becomes 'N' (chess abbreviation).
'celebrated' becomes 'famous' (synonyms).
'i'+'n'+'famous'='INFAMOUS'
'but' acts as a link.
(Other definitions for infamous that I've seen before include "Dishonourable" , "Disreputable" , "Disgraceful" , "Morally bad, shocking" , "Notorious, having evil reputation" .)