Left cuts excellent boxer (3)
I believe the answer is:
ali
'boxer' is the definition.
(Muhammad Ali)
'left cuts excellent' is the wordplay.
'left' becomes 'L' (common abbreviation).
'cuts' indicates putting letters inside (some letters cut their way into other letters).
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'l' inserted inside 'ai' is 'ALI'.
(Other definitions for ali that I've seen before include "Cassius Clay became Muhammad ..." , "Muhammad or Baba?" , "Muhammad's son-in-law" , "girl shortly" , "Old fighter" .)