"The Greatest” (3)
'the greatest?' is the definition.
(Muhammad Ali, known as the Greatest)
This is the entire clue.
(Other definitions for ali that I've seen before include "Mohammed . . . (boxer)" , "Ring' specialist" , "A champ'" , "Muhammad ... (former fighter)" , "Former boxing champion" .)
(Other definitions for nth that I've seen before include "To the -- degree" , "Umpteenth" .)
(Another definition for muhammad that I've seen is " Founder of the Islamic faith".)