Sound defeat of horse chestnut used by children (6)
I believe the answer is:
conker
'horse chestnut used by children' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'sound defeat' is the wordplay.
'sound' shows a homophone (sound like).
'defeat' becomes 'conquer' (to conquer is to defeat or beat).
'conquer' is a homophone of 'CONKER'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for conker that I've seen before include "Chestnut on a string" , "Nut of the horse chestnut tree" , "Chestnut fruit" , "Seed of horse chestnut" , "Horse chestnut's fruit" .)