Left again the day after (3)
I believe the answer is:
red
'left' is the definition.
(red can informally mean communist)
'again the day after' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'the day' becomes 'd' (abbreviation for day).
'after' says to put letters next to each other.
're'+'d'='RED'
(Other definitions for red that I've seen before include "Very angry, seeing this" , "Bloodshot" , "14 -- breed of 4" , "Colour of Merlot wine" , "Merchant flag" .)