Arrestingly reactionary (3)
I believe the answer is:
red
'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" .)