Common defense (5)
I believe the answer is:
alibi
'common defense' 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 "Making excuse" , "Proof of not being at the scene of the crime" , "Evidence one was elsewhere at time of crime" , "it may help you to get off" , "Plea of absence" .)