Greatest boxer briefly penetrated defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
'greatest boxer briefly penetrated' is the wordplay.
I cannot quite understand how this works, but
'boxer' could be 'ali' (Muhammad Ali) and 'ali' is located in the answer.
The remaining letters 'bi' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for alibi that I've seen before include "It shows one wasn't at crime scene" , "Timely get-out" , "that gets him off the hook?" , "Accused person's defence" , "Reason often given to exculpate someone" .)