Permit His Excellency, a knight, being oblivious (7)
I believe the answer is:
lethean
'oblivious' is the definition.
(thesaurus)
'permit his excellency a knight' is the wordplay.
'permit' becomes 'let' ('let' can be a synonym of 'permit').
'his excellency' becomes 'he'.
'knight' becomes 'n' (chess).
'let'+'he'+'a'+'n'='LETHEAN'
'being' is the link.
(Other definitions for lethean that I've seen before include "Making one forget" , "to make elephant forget?" , "Preventing recall" , "Causing forgetfulness and oblivion" , "wiping memory" .)