Guy looking up, dismissing a tree (3)
I believe the answer is:
elm
'tree' is the definition.
(elm is a kind of tree)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for elm that I've seen before include "Species of tree" , "Tree of the genus Ulmus" , "Mel's tree" , "one has a trunk and leaves" , "Tall shade tree" .)