Revolutionary song about old judge, a Red (5)
I believe the answer is:
rioja
'a red' is the definition.
(I know that Rioja is a type of red)
'revolutionary song about old judge' is the wordplay.
'revolutionary' shows that the letters should be reversed in order.
'song' becomes 'air' (I've seen this in another clue).
'about' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'judge' becomes 'j' (abbreviation for judge).
'air' in reverse letter order is 'ria'.
'o'+'j'='oj'
'ria' enclosing 'oj' is 'RIOJA'.
(Other definitions for rioja that I've seen before include "wine region" , "Spanish red wine" , "North Spanish wine" , "it comes from grapevine" , "Spanish wine type" .)