Starchy tuber small and sweet (4)
I believe the answer is:
spud
'starchy tuber' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'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'
Can you help me to learn more?
(Other definitions for spud that I've seen before include "Potato (informal)" , "short, stocky person" , "One could be in jacket" , "King Edward, say (slang)" , "The common potato" .)