Mighty trees (4)
I believe the answer is:
oaks
'mighty trees' 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 "material for shipbuilding" , "Trees of the genus Quercus" , "Soak up with trees" , "Epsom Classic race" , "Acorn producers" .)