Crossword Genius

This precursor of flood proving ruinously expensive (8,3,4)

Ross

I believe the answer is:

breaking the bank

I'm a little stuck... Click here to teach me more about this clue!

'proving ruinously expensive' is the definition.
(something that breaks the bank is expensive)

I can't explain the rest of the clue.

Can you help me to learn more?

I've seen this clue in The Guardian.
Want a hint initially instead of a full solution? Install my app