Vegetable seconds with a little dessert (4)
I believe the answer is:
spud
'vegetable' is the definition.
(spud is a kind of vegetable)
'seconds with a little dessert' is the wordplay.
'seconds' becomes 's'.
'with' means one lot of letters go next to another.
'a little dessert' becomes 'pud' (short for pudding).
's'+'pud'='SPUD'
(Other definitions for spud that I've seen before include "Murphy" , "one's eyed" , "The popular potato" , "Potato (colloq.)" , "Spade; potato (slang)" .)