Knight noble, Yankee not quite (6)
I believe the answer is:
nearly
'not quite' is the definition.
(nearly can mean not quite or almost)
'knight noble yankee' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'noble' becomes 'earl' (earl is a kind of noble).
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'n'+'earl'+'y'='NEARLY'
(Other definitions for nearly that I've seen before include "Give out a bright light" , "parsimoniously" , "Noon at eleven fifty nine am" , "Almost, well-nigh" , "Almost, not quite" .)