The most common and most likely answer to this crossword clue is the 16 letter word ONE-ARMED-BANDIT
O
N
E
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A
R
M
E
D
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B
A
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I
T
Buddy explains!
The straight part
We know that "standing on the casino floor" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case standing on the casino floor = one-armed-bandit.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.