Excuse getting both Caliban and Cain out of prison (5)

I believe the answer is:

alibi

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'excuse' is the definition.
(excuse given by a suspect in an investigation)

I don't understand how the rest of the clue works.

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(Other definitions for alibi that I've seen before include "Cover story" , "Timely plea?" , "Claim put to court" , "Proof of not being at crime site" , "Proof one was elsewhere when crime was committed" .)

I've seen this clue in The Independent.

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