Bananas excellent served with some tea? (8)
I believe the answer is:
crackpot
'bananas' is the definition.
(both can informally mean mad)
'excellent served with some tea?' is the wordplay.
'excellent' becomes 'crack' (the most excellent, as in a 'crack team').
'served with' says to put letters next to each other.
'some tea?' becomes 'pot' (pot of tea).
'crack'+'pot'='CRACKPOT'
(Other definitions for crackpot that I've seen before include "Eccentric; impractical" , "Mad" , "Foolish" , "Eccentric (colloq.)" , "Idiot" .)