Convert smallest amount for vegetable (6)
I believe the answer is:
turnip
'for vegetable' is the definition.
(turnip is a kind of vegetable)
'convert smallest amount' is the wordplay.
'convert' becomes 'turn' (synonyms).
'smallest amount' becomes 'ip' (I am not sure about this - if you are sure you should believe this answer much more).
'turn'+'ip'='TURNIP'
Can you help me to learn more?
(Other definitions for turnip that I've seen before include "Large-rooted vegetable related to the cabbage" , "what you eat?" , "food item" , "Rip nut up for a vegetable" , "'Root vegetable, yellow or white (6)'" .)