Fruit of the oak (5)
I believe the answer is:
acorn
'fruit of the oak' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for acorn that I've seen before include "Potentially future tree" , "Dirt on a liquid surface" , "oak's fruit" , "From which a tall tree may grow" , "old computer?" .)