Root vegetable taking a month to come up (3)
I believe the answer is:
yam
'root vegetable' is the definition.
(yam is a kind of root vegetable)
'a month to come up' is the wordplay.
'a month' becomes 'may' (May is an example).
'to come up' says the letters should be written backwards.
'may' in reverse letter order is 'YAM'.
'taking' acts as a link.
(Other definitions for yam that I've seen before include "A starchy tuber" , "Tuberous vegetable" , "US sweet-potato" , "Tropical tuber" , "US sweet potato" .)