Substitute player knocked unconscious, part of boot on youngster's head (10)
I believe the answer is:
understudy
'substitute' is the definition.
(I know that understudy is a type of substitute)
'unconscious part of boot on youngster's head' is the wordplay.
'unconscious' becomes 'under' (as in under anaesthetic).
'part of boot on youngster's head' becomes 'study' (I can't justify this - if you can you should believe this answer much more).
'under'+'study'='UNDERSTUDY'
'player knocked' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for understudy that I've seen before include "emergency role" , "one's hoping to get on" , "Replacement" , "reserve?" , "Stage stand-in" .)