Liquid added to hot water to make it foam (6,4)

I believe the answer is:

bubble bath

(Other definitions for bubble bath that I've seen before include "Foam-producing substance" , "It adds foam and scent to water" , "agent foaming" , "Lathering agent for ablutions" , "Relaxing soapy additive to water" .)

I've seen this clue in The Guardian.

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