Posh knight drunk: not exactly bright (5)
I believe the answer is:
unlit
'not exactly bright' is the definition.
(I've seen this before)
'posh knight drunk' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'knight' becomes 'N' (chess abbreviation).
'drunk' becomes 'lit' (slang term).
'u'+'n'+'lit'='UNLIT'
(Other definitions for unlit that I've seen before include "Not yet fired up" , "not exactly bright" , "With no lighting" , "such as dark alley?" , "Without illumination" .)