Dessert on small side dish? (4)
I believe the answer is:
spud
'side dish?' is the definition.
(informal term for potato)
'dessert on small' is the wordplay.
'dessert' becomes 'pud' (short for pudding).
'on' says to put letters next to each other.
'small' becomes 's' (abbreviation - e.g. clothes size).
'pud' put after 's' is 'SPUD'.
(Other definitions for spud that I've seen before include "Potato (colloq.)" , "short, stocky person" , "The popular potato" , "Potato (informal)" , "Weed spade" .)