A politician one making excuse (5)
I believe the answer is:
alibi
'making excuse' is the definition.
(I've seen this before)
'a politician one' is the wordplay.
'politician' becomes 'lib'.
'one' becomes 'i' (Roman numeral).
'a'+'lib'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "Evidence of (or excuse for) being somewhere else at the time" , "Sort of explanation" , "Justification" , "A claim one was elsewhere when crime took place" , "can offer a valid defence against most charges" .)