Fun for eg young Boris, having surprisingly low mental age (4,4,4)

I believe the answer is:

eton wall game

I'm a little stuck... Click here to teach me more about this clue!

'fun for eg young boris' is the definition.
I know nothing about this answer so I can't judge whether this works.

'surprisingly low mental age' is the wordplay.
'surprisingly' indicates anagramming the letters (a surprising ordering of the letters).
'low'+'mental'+'age'='lowmentalage'
'lowmentalage' anagrammed gives 'ETON WALL GAME'.

'having' acts as a link.

Can you help me to learn more?

(Another definition for eton wall game that I've seen is " School competition".)

I've seen this clue in The Guardian.

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