One not bright or amusing when drunk (9)
I believe the answer is:
ignoramus
'one not bright' is the definition.
(stupid person)
'or amusing when drunk' is the wordplay.
'when drunk' is an anagram indicator (a drunken or disorderly form of the letters).
'or'+'amusing'='oramusing'
'oramusing' with letters rearranged gives 'IGNORAMUS'.
(Other definitions for ignoramus that I've seen before include "Fool" , "One could be blissful it's said" , "Simpleton" , "Ignorant person" , "One lacking knowledge" .)