Scoff vegetables, no good on reflection (5)
I believe the answer is:
sneer
'scoff' is the definition.
(I've seen this before)
'vegetables no good on reflection' is the wordplay.
'vegetables' becomes 'greens' (I've seen this before).
'no' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'on reflection' shows that the letters should be reversed in order.
'greens' with 'g' taken out is 'reens'.
'reens' in reverse letter order is 'SNEER'.
(Other definitions for sneer that I've seen before include "Dig" , "Contemptuous look" , "A scornful expression or remark" , "Speak contemptuously" , "Speak scornfully" .)