Foolish joke, better on reflection (8)
I believe the answer is:
crackpot
'foolish' is the definition.
(I've seen this before)
'joke better on reflection' is the wordplay.
'joke' becomes 'crack' (a crack is a joke or quip).
'better' becomes 'top' (top can mean to better or outdo).
'on reflection' shows that the letters should be reversed in order.
'top' written backwards gives 'pot'.
'crack'+'pot'='CRACKPOT'
(Other definitions for crackpot that I've seen before include "Eccentric; impractical" , "Harebrained type" , "Nut" , "Informal eccentric like damaged utensil" , "Damage utensil for informally eccentric person" .)