Crossword Genius

At “hair of the dog,” become angry (7)

Ross

I believe the answer is:

bristle

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'at hair' is the definition.
'bristle' can be an answer for 'hair' (bristle is a kind of hair). I'm not certain of the 'at' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for bristle that I've seen before include "A short stiff hair, as in beard" , "Get angry" , "Whisker" , "React angrily" , "A stiff hair on chin" .)

I've seen this clue in the Evening Standard.
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