One who perjures or frees war criminal (10)

'frees war criminal' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'one who perjures or' is the wordplay.
I cannot really see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
'or' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?