Politician protected by first-class defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
'politician protected by first-class' is the wordplay.
'politician' becomes 'lib'.
'protected by' indicates putting letters inside (inserted letters are held or protected).
'first class' becomes 'ai' (A1 - top quality).
'lib' going inside 'ai' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "It shows one wasn't at crime scene" , "that gets him off the hook?" , "Defence that one was somewhere else at the time of a crime" , "Proof of being elsewhere at the time of a crime." , "Excuse for failure" .)