A politician with one excuse (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'a politician with one' is the wordplay.
'politician' becomes 'lib'.
'with' says to put letters next to each other.
'one' becomes 'I' (Roman numeral).
'a'+'lib'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "Claim put to court" , "Defence of being elsewhere at time of crime" , "Cover story" , "Legal excuse" , "Justification" .)