Acorn droppers (4)
I believe the answer is:
'acorn droppers' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oaks that I've seen before include "Acorn producers" , "Horse race - trees" , "Mighty ones from little acorns grow" , "Epsom Classic race" , "Hardwood trees" .)