Small sweet tuber (4)
I believe the answer is:
spud
'tuber' 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 "one's eyed" , "King Edward, say (slang)" , "One could be in jacket" , "The common potato" , "Potato (colloq.)" .)