Oak or ash, for example (4)
I believe the answer is:
tree
This clue is a double definition.
'oak' is the first definition.
(I've seen this before)
'ash for example' is the second definition.
(ash is a kind of tree)
(Other definitions for tree that I've seen before include "Birch, say" , "Older actor" , "trod the boards" , "Theme" , "Shoe __ or family __" .)