Plant stem (5)
(Other definitions for tuber that I've seen before include "Underground stem, root" , "Fleshy underground part of a stem, often used as food" , "Rhizome, as of potato" , "Potato, for example" , "or still vegetating?" .)
'plant stem' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for stalk that I've seen before include "Follow prey stealthily" , "Shadow" , "Follow sneakily" , "Stem; pursue stealthily" , "Quietly pursue" .)