American politician one making excuse (5)
I believe the answer is:
alibi
'making excuse' is the definition.
(I've seen this before)
'american politician one' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'politician' becomes 'lib'.
'one' becomes 'i' (Roman numeral).
'a'+'lib'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "Answer to charge - was somewhere else" , "can offer a valid defence against most charges" , "Evidence one was elsewhere at time of crime" , "Timely plea?" , "Excuse for failure" .)