Exonerating excuse (5)
I believe the answer is:
alibi
'exonerating excuse' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alibi that I've seen before include "Answer to charge - was somewhere else" , "Defence against accusation - was somewhere else" , "It shows one wasn't at crime scene" , "Courtroom evidence" , "Reason often given to exculpate someone" .)