A child of four (10)
I believe the answer is:
Here is my best explanation:
'a child of four' is the definition.
'child of four' is the wordplay.
I cannot really see how this works, but
'child' could be 'quad' (I have seen 'Child of four ' mean 'quad' so perhaps 'child' could also mean 'quad') and 'quad' is found in the answer.
This explanation may well be incorrect...
(Another definition for quadruplet that I've seen is "part of big delivery".)