Gathering time for root vegetable (4)
I believe the answer is:
beet
'root vegetable' is the definition.
(beet is a kind of root vegetable)
'gathering time' is the wordplay.
'gathering' becomes 'bee' (bee is a kind of gathering).
'time' becomes 't'.
'bee'+'t'='BEET'
'for' is the link.
(Other definitions for beet that I've seen before include "Plant grown for sugar" , "Root, maybe source of sugar" , "Crop plant, sugar source" , "E.g. chard, mangelwurzel" , "Root used as a source of sugar" .)