Child of Elizabeth II on small and sweet horse (9)
I believe the answer is:
tweenager
'child' is the definition.
(I have seen 'Precocious child' mean 'tweenager' so perhaps 'child' could also mean 'tweenager')
'elizabeth ii on small and sweet horse' is the wordplay.
'elizabeth ii' becomes 'er' (I can't explain this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'small and sweet' becomes 'twee' (I've seen this before).
'horse' becomes 'nag' (term for an old horse).
'twee'+'nag'='tweenag'
'er' put after 'tweenag' is 'TWEENAGER'.
'of' is the link.
Can you help me to learn more?
(Other definitions for tweenager that I've seen before include "Possibly a ten-year-old" , "Youngster" , "Precocious child" , "exasperating twelve-year-old" , "adolescent" .)