A constituent of the water we all need (6)

I believe the answer is:

oxygen

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'a constituent' is the definition.
(I know that oxygen is a type of chemical element)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for oxygen that I've seen before include "Gaseous element supporting combustion" , "Essential gas for most plants and animals" , "The second most common element on earth" , "Colourless, odourless, tasteless non-flammable gas" , "Gas discovered by Joseph Priestley, d. 1804" .)

I've seen this clue in the Evening Standard.

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