Water a water mattress won’t have! (6)

I believe the answer is:

spring

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'water' is the definition.
(spring water is a kind of water)

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

(Other definitions for spring that I've seen before include "Leap, well" , "Shoot forth" , "past February is it?" , "Leap; season" , "split" .)

I've seen this clue in the Evening Standard.

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