Politician taken in by excellent excuse (5)
I believe the answer is:
alibi
'excuse' is the definition.
(alibiing is a kind of excusing)
'politician taken in by excellent' is the wordplay.
'politician' becomes 'lib'.
'taken in by' indicates putting letters inside.
'excellent' becomes 'ai'.
'lib' placed within 'ai' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Justification" , "can offer a valid defence against most charges" , "Legal proof of innocence" , "Story confirming innocence" , "Excuse for not doing something" .)