A party has one defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
'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 "Legal defence - was elsewhere" , "Cover story" , "Reason often given to exculpate someone" , "A claim one was elsewhere when crime took place" , "Defence against accusation - was somewhere else" .)