Horse chestnut (6)
I believe the answer is:
conker
'horse chestnut' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for conker that I've seen before include "Seed of horse chestnut" , "Nut of the horse chestnut tree" , "hard nut" , "Chestnut fruit" , "Horse chestnut's fruit" .)