The most common and most likely answer to this crossword clue is the 3 plus 4 plus 4 plus 4 letter word THE ETON WALL GAME
I've seen this crossword clue in The Daily Telegraph
T
H
E
E
T
O
N
W
A
L
L
G
A
M
E
Buddy explains!
The straight part
We know that "sport with posh boys" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sport with posh boys = the eton wall game.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.