Crossword Genius

One relies on them to contain water (5)

Ross

I believe the answer is:

banks

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'one relies' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for banks that I've seen before include "Relies" , "Financial institutions" .)

I've seen this clue in the Evening Standard.
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