Eccentric, but excellent, casserole (8)
I believe the answer is:
crackpot
'eccentric' is the definition.
(I know that crackpot is a type of eccentric)
'excellent casserole' is the wordplay.
'excellent' becomes 'crack' (the most excellent, as in a 'crack team').
'casserole' becomes 'pot' (casserole is a type of cooking pot).
'crack'+'pot'='CRACKPOT'
'but' acts as a link.
(Other definitions for crackpot that I've seen before include "Crazy or impractical person" , "Informal eccentric like damaged utensil" , "Odd" , "Eccentric (colloq.)" , "Idiot" .)