Mighty ones grow from little acorns (4)
I believe the answer is:
oaks
'mighty ones grow from little acorns' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for oaks that I've seen before include "material for shipbuilding" , "Hardwood trees" , "Acorn trees" , "Epsom Classic race" , "Mighty ones from little acorns grow" .)