The greatest in equality? (3)
I believe the answer is:
ali
'the greatest' is the definition.
(film about Muhammad Ali)
'in equality?' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ALI' can be found hidden inside 'quality'.
(Other definitions for ali that I've seen before include "Muhammad ..., famous boxer" , ""The Greatest" boxer" , "Muhammad ... (former fighter)" , "Late boxer" , "Muhammad - - , legendary boxer" .)