Quantity of liquid a fish needs to stay alive (4)

I believe the answer is:

gill

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'quantity' is the definition.
(gill is a kind of quantity)

I don't understand the remainder of the clue.

(Other definitions for gill that I've seen before include "Fish's breathing apparatus" , "Small liquid measure" , "Respiratory organ of fish" , "Organ for breathing under water" , "Eric ---, sculptor; breathing organ" .)

I've seen this clue in the Evening Standard.

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