Defence of being elsewhere (5)
I believe the answer is:
alibi
'defence 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 "Legal proof of innocence" , "Courtroom evidence" , "Evidence one was elsewhere at time of crime" , "I bail (anag.)" , "Justification" .)