A hasty sensitive reaction? (4)

I believe the answer is:

rash

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'a hasty' is the definition.
(I've seen this in another clue)

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

(Other definitions for rash that I've seen before include "complaint" , "Daredevil" , "Lacking in caution" , "Hives" , "Red area on skin" .)

I've seen this clue in the Evening Standard.

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