The most common and most likely answer to this crossword clue is the 6 plus 4 plus 5 letter word DOUBLE YOUR MONEY
I've seen this crossword clue in The Daily Telegraph
D
O
U
B
L
E
Y
O
U
R
M
O
N
E
Y
Buddy explains!
The straight part
We know that "old quiz show" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old quiz show = double your money.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.