What does the cryptic crossword clue "Fun for eg young Boris, having surprisingly low mental age (4,4,4)" mean?

The wordplay uses an anagram. anagram of LOW + MENTAL + AGE = ETONWALLGAME The answer is ETONWALLGAME.

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

Guardian Cryptic #29187 | 10 Down | 2023-09-28 flower