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