Old-style politician involved in excellent excuse (5)
I believe the answer is:
alibi
'excuse' is the definition.
(alibiing is a kind of excusing)
'old-style politician involved in excellent' is the wordplay.
'old-style politician' becomes 'lib' (Liberal).
'involved in' indicates putting letters inside.
'excellent' becomes 'ai'.
'lib' placed within 'ai' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Suspect's excuse" , "Timely get-out" , "Excuse, defence" , "Timely excuse" , "Proof of being elsewhere at the time of a crime." .)