The most common and most likely answer to this crossword clue is the 5 plus 3 plus 4 letter word BREAK THE BANK
B
R
E
A
K
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B
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K
More clues leading to the result BREAK THE BANK
Buddy explains!
The straight part
We know that "Cause insupportable costs/as flooded river may?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cause insupportable costs/as flooded river may? = break the bank.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.