Left cuts first-rate boxer (3)
I believe the answer is:
ali
'boxer' is the definition.
(Muhammad Ali)
'left cuts first-rate' is the wordplay.
'left' becomes 'l' (common abbreviation).
'cuts' means one lot of letters goes inside another (some letters cut their way into other letters).
'first-rate' becomes 'ai' (resembles A1, informal term for excellent).
'l' put within 'ai' is 'ALI'.
(Other definitions for ali that I've seen before include "Muhammad - - , legendary boxer" , "Ring' specialist" , "Ringmaster" , "Champion of ring" , "Lepidopterous floater, apian stinger, d. 2016" .)