Excuse from politician sitting in first-class (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'politician sitting in first-class' is the wordplay.
'politician' becomes 'lib'.
'sitting in' is an insertion indicator.
'first class' becomes 'ai' (A1 - top quality).
'lib' going within 'ai' is 'ALIBI'.
'from' is the link.
(Other definitions for alibi that I've seen before include "Defence that one was somewhere else at the time of a crime" , "Excuse, in law" , "It shows one wasn't at crime scene" , "Excuse - plea of being elsewhere" , "Cover story" .)