Underground stem (7)
I believe the answer is:
rhizome
'underground stem' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rhizome that I've seen before include "Subterranean plant stem" , "Type of rootstock" , "that supplies stock" , "Root" , "Underground plant stem producing roots and shoots - zero him (anag)" .)