Noted horse used in breeding (3)
I believe the answer is:
egg
'in breeding' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'noted horse' is the wordplay.
I cannot really see how this works, but
'horse' could be 'gg' (geegee or GG is a childish term for a horse) and 'gg' is found within the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'used' acts as a link.
Can you help me to learn more?
(Other definitions for egg that I've seen before include "On the face, its an embarrassment" , "Incite; ovum" , "Laid item" , "Reproductive unit" , "Reproductive part" .)