Small sweet potato (4)
I believe the answer is:
spud
'potato' is the definition.
(spud can informally mean potato)
'small sweet' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'sweet' becomes 'pud' (pud is a kind of sweet).
's'+'pud'='SPUD'
(Other definitions for spud that I've seen before include "King Edward, say (slang)" , "The popular potato" , "Starchy bit of food" , "Spade; potato (slang)" , "One might have an eye" .)