Root vegetable producing sugar (4)
I believe the answer is:
beet
'root vegetable producing sugar' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for beet that I've seen before include "Plant grown for sugar" , "An edible root plant" , "Root, maybe source of sugar" , "Source of food" , "E.g. chard, mangelwurzel" .)