Lecturer punches excellent boxer (3)
I believe the answer is:
ali
'boxer' is the definition.
(Muhammad Ali)
'lecturer punches excellent' is the wordplay.
'lecturer' becomes 'l' (abbreviation for lecturer).
'punches' means one lot of letters goes inside another (some letters punch a hole in others).
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'l' inserted within 'ai' is 'ALI'.
(Other definitions for ali that I've seen before include "Mohammad . . ., . . . Hewson" , "Former boxing champion" , "US heavyweight boxer" , "girl shortly" , "Late boxer" .)