An out-of-place plea of innocence (5)
I believe the answer is:
alibi
'an out' is the definition.
(I've seen this in another clue)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for alibi that I've seen before include "Defence against accusation - was seen elsewhere at the time" , "can offer a valid defence against most charges" , "Proof of being elsewhere at the time of a crime." , "Defence against charge - was seen elsewhere" , "Legal term from the Latin 'elsewhere'" .)