Elm or oak (4)
I believe the answer is:
tree
I believe this clue is a double definition.
'elm' is the first definition.
(I've seen this in another clue)
'oak' is the second definition.
(I've seen this before)
(Other definitions for tree that I've seen before include "Root-and-branch plant?" , "Large plant with trunk" , "Source of wood" , "eg Rowan" , "Big plant" .)