Root vegetable needs month to come up (3)
I believe the answer is:
yam
'root vegetable' is the definition.
(yam is a kind of root vegetable)
'month to come up' is the wordplay.
'month' becomes 'may' (May is an example).
'to come up' is a reversal indicator.
'may' back-to-front is 'YAM'.
'needs' acts as a link.
(Other definitions for yam that I've seen before include "May be a foreign potato" , "Tropical root vegetable" , "Tropical climber" , "May be a sweet spud" , "Tropical tuber" .)