Horse's young footballers (5)
I believe the answer is:
colts
'footballers' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'horse's young' is the wordplay.
'horse' becomes 'colt' (I've seen this before).
'young' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'colt'+'s'='COLTS'
Can you help me to learn more?
(Other definitions for colts that I've seen before include "Young stallions" , "Young male horses" , "young players" .)