Vegetable that’s small and sweet (4)
I believe the answer is:
spud
'vegetable' is the definition.
(spud is a kind of vegetable)
'small and sweet' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' says to put letters next to each other.
'sweet' becomes 'pud' (pud is a kind of sweet).
's'+'pud'='SPUD'
'that's' acts as a link.
(Other definitions for spud that I've seen before include "The common potato" , "One might have an eye" , "Murphy" , "Commoner's King Edward?" , "one's eyed" .)