A party has one excuse (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'a party has one' is the wordplay.
'party' becomes 'lib' (abbreviation for Liberal, uk political party).
'has' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'a'+'lib'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "Defence in court" , "Excuse, defence" , "Justification" , "Sort of explanation" , "can offer a valid defence against most charges" .)