Excuse of being elsewhere (5)
I believe the answer is:
alibi
'excuse of being elsewhere' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for alibi that I've seen before include "Defence used by an accused person" , "Excuse, in law" , "Defence of being elsewhere at time of crime" , "Story confirming innocence" , "Proof of absence" .)