Tuber (6)
(Other definitions for root that I've seen before include "Turn up with the snout." , "Basic source or origin" , "burrow" , "Underground part of a plant" , "cheer" .)
'tuber' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for bulb that I've seen before include "Onion, perhaps" , "Source of flower or light" , "daffodil here" , "Corm; electric light" , "Tulip, for example" .)
'tuber' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for potato that I've seen before include "Edible tuber; hole in stocking (colloq.)" , "It's often baked" , "Common vegetable" , "The humble spud" , "Maris Piper?" .)