A politician with one excuse for failure (5)
I believe the answer is:
alibi
'excuse for failure' is the definition.
(I've seen this before)
'a politician with one' is the wordplay.
'politician' becomes 'lib'.
'with' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'a'+'lib'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "Timely excuse" , "Defence against accusation - was seen elsewhere at the time" , "Accused person's defence" , "Claim put to court" , "Plea of absence" .)