Crossword Genius

Arrestingly reactionary (3)

Ross

I believe the answer is:

red

I'm a little stuck... Click here to teach me more about this clue!

'reactionary' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?

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

Can you help me to learn more?

(Other definitions for red that I've seen before include "could be a Marxist" , "Primary colour" , "Crimson" , "Colour of garnets" , "The colour of the former Russian army" .)

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