EastEnders, for instance, use it in the washing (4)

I believe the answer is:

soap

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'washing' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for soap that I've seen before include "Cleaning agent" , "Washing aid" , "Carbolic, perhaps" , "TV programme; wash" , "Cleansing agent" .)

I've seen this clue in the Evening Standard.

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