Fleshy underground stem (5)
I believe the answer is:
tuber
'fleshy underground stem' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tuber that I've seen before include "Fleshy underground root" , "Brute (anag)" , "Fleshy underground part of a stem, often used as food" , "Root eg of potato" , "Thickened underground stem" .)