Scoff vegetables, no good on reflection (5)
I believe the answer is:
Here is my best explanation:
'scoff vegetables' is the definition.
'sneer' can be an answer for 'scoff' (thesaurus). I am not sure about the 'vegetables' bit.
'no good on reflection' is the subsidiary indication.
'no good' becomes 'sne' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' becomes 're' ('on' can be mean 'with reference to').
'reflection' shows that the letters should be reversed in order.
're' backwards is 'er'.
(Other definitions for sneer that I've seen before include "Scornful expression", "Speak contemptuously", "Contemptuous or mocking smile", "Derisive remark", "Show contempt with scornful expression or words", "Curl one's lip", "Dig", "Taunt", "Contemptuous remark" and "sound scornful".)