Red — Irish county Marxist, ultimately (6)
I believe the answer is:
claret
'red' is the definition.
(claret is a kind of red)
'irish county marxist ultimately' is the wordplay.
'irish county' becomes 'clare'.
'ultimately' indicates one should take the final letters.
The last letter of 'marxist' is 't'.
'clare'+'t'='CLARET'
(Other definitions for claret that I've seen before include "Deep red colour" , "Shade of red" , "Irish county almost makes a red wine" , "vessel might contain this?" , "Deep purplish colour" .)