On one, on and start sorting vegetables (6)
I believe the answer is:
onions
'vegetables' is the definition.
(I know this)
'on one on and start sorting' is the wordplay.
'one' becomes 'i' (Roman numeral).
'and' is a charade indicator (letters next to each other).
'start' says to take the initial letters.
The initial letter of 'sorting' is 's'.
'on'+'i'+'on'+'s'='ONIONS'
(Other definitions for onions that I've seen before include "Such may make one weep" , "English cricketer" , "Experts know theirs" , "Famous lexicographer" , "Pungent bulbs (6)." .)