The most common and most likely answer to this crossword clue is the 14 letter word JUMPSÂ ONÂ BOARD
I've seen this crossword clue in The Guardian
J
U
M
P
S
Â
O
N
Â
B
O
A
R
D
Buddy explains!
The straight part
We know that "What a game knight does" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what a game knight does = jumps on board.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.