A party with one defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(defence given to police by a suspect)
'a party with one' is the wordplay.
'party' becomes 'lib' (abbreviation for Liberal, uk political party).
'with' says to put letters next to each other.
'one' becomes 'I' (Roman numeral).
'a'+'lib'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "Proof of absence" , "Defence that one was somewhere else at the time of a crime" , "Excuse, defence" , "Making excuse" , "Excuse, in law" .)