Excuse one replacing a graduate in panto role (5)
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'one replacing a graduate in panto role' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'li' (I've seen this in other clues) and 'li' is found in the answer.
'a' is within the answer.
The remaining letters 'bi' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for alibi that I've seen before include "Legal excuse", "it may help you to get off", "can offer a valid defence against most charges", "Evidence one was not at crime scene", "in order to prove I was elsewhere".)