Tree over in arboretum, leafy (3)
I believe the answer is:
elm
'tree' is the definition.
(elm tree is a kind of tree)
'over in arboretum leafy' is the wordplay.
'over' says the letters should be written in reverse.
'in' says the answer is hidden in the clue.
'mle' is hidden in the letters of 'arboretum leafy'.
'mle' written backwards gives 'ELM'.
(Other definitions for elm that I've seen before include "Leaving body" , "Tree for a Cockney tiller" , "Furniture wood" , "Strange tree for Mel" , "Wood in pelmet" .)