Oak, eg (4)
I believe the answer is:
tree
'oak eg' is the definition.
(oak is a kind of tree)
This is the entire clue.
(Other definitions for tree that I've seen before include "Elm, perhaps" , "Ash, chestnut, or araucaria, for example" , "Box or bay maybe?" , "elder, maybe" , "Shakespearean actor" .)