Does it call for hasty treatment? (4)

I believe the answer is:

rash

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'treatment?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

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

Can you help me to learn more?

(Other definitions for rash that I've seen before include "Careless" , "Hives" , "Impulsive - eruption" , "Skin redness" , "sign of irritation?" .)

I've seen this clue in the Evening Standard.

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