Mix rats, mix a communist (7)
I believe the answer is:
marxist
'a communist' is the definition.
(Marxist is an example)
'mix rats mix' is the wordplay.
'mix' indicates anagramming the letters.
'rats'+'mix'='ratsmix'
'ratsmix' anagrammed gives 'MARXIST'.
(Other definitions for marxist that I've seen before include "Revolutionary" , "left-wing" , "Red" , "A leftie once" , "left-winger" .)