Potato, small and sweet (4)
I believe the answer is:
spud
'potato' is the definition.
(spud can informally mean potato)
'small and sweet' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' is a charade indicator (letters next to each other).
'sweet' becomes 'pud' (pud is a kind of sweet).
's'+'pud'='SPUD'
(Other definitions for spud that I've seen before include "veg" , "Charlotte perhaps" , "The popular potato" , "Tattie" , "King Edward, say (slang)" .)