The most common and most likely answer to this crossword clue is the 6 plus 4 letter word CLOSED BOOK
I've seen this crossword clue in The Daily Telegraph
C
L
O
S
E
D
B
O
O
K
Buddy explains!
The straight part
We know that "Stopped taking bets/I don’t understand it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case stopped taking bets/i don’t understand it = closed book.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.