Grown-up acorns (4)
I believe the answer is:
oaks
'grown-up acorns' 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" , "Epsom Classic race" , "Acorn trees" , "Soak up with trees" , "Trees of the genus Quercus" .)