Positional defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for alibi that I've seen before include "Timely excuse" , "I bail (anag.)" , "Excuse - plea of being elsewhere" , "Defence against accusation - was seen elsewhere at the time" , "Legal term from the Latin 'elsewhere'" .)