Excuse one replacing a graduate in panto role (5)
I believe the answer is:
alibi
'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 proof of innocence" , "Evidence of (or excuse for) being somewhere else at the time" , "Defence in court" , "Defence that one was somewhere else at the time of a crime" , "Excuse, defence" .)