Revolutionary arrested by Confederates retreating (3)
I believe the answer is:
red
'revolutionary' is the definition.
(red can informally mean communist)
'arrested by confederates retreating' is the wordplay.
'arrested by' indicates the answer is hidden within the clue (the answer is held or arrested by the clue).
'retreating' says the letters should be written backwards.
'der' can be found hidden inside 'confederates'.
'der' written backwards gives 'RED'.
(Other definitions for red that I've seen before include "eg Cardinal" , "14 -- breed of 4" , "A colourful communist" , "The colour of the former Russian army" , "lefty" .)