Trees (4)
'trees' is the definition.
(elm is a kind of tree)
This is the entire clue.
(Other definitions for elms that I've seen before include "Trees (of late, prone to disease)" , "Deciduous trees with rough serrated leaves" , "may have many leaves" , "rugged feature of country churchyard" , "Trees sound like Cockney steering devices" .)
(Other definitions for oaks that I've seen before include "Epsom Classic race" , "Acorn producers" , "Acorn trees" , "Hardwood trees" , "material for shipbuilding" .)