Legal defence (5)
I believe the answer is:
'legal defence' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for alibi that I've seen before include "Defence of being elsewhere at time of crime" , "Claim put to court" , "Court evidence showing one was elsewhere" , "Lawyer's weapon, Latin for 'elsewhere'" , "Legal defence proving innocence" .)