Old king who's 50 per cent cockney 100 per cent Marxist? (6)
I believe the answer is:
alfred
'old king' is the definition.
(Alfred the Great)
'50 per cent cockney 100 per cent marxist?' is the wordplay.
'50 per cent cockney' becomes ''alf' ('half' - I've seen this before - said with the 'H' dropped - a cockney accent.).
'100 per cent marxist?' becomes 'red' (red can informally mean left-wing).
'alf'+'red'='ALFRED'
'who's' is the link.
(Other definitions for alfred that I've seen before include "King of Wessex, known as "the Great"" , "King after Ethelred" , "Ninth century king of Wessex" , "- - Hitchcock" , "Great English king" .)