Excuse of a party, formerly one (5)
I believe the answer is:
alibi
'excuse' is the definition.
(excuse given by a suspect in an investigation)
'a party formerly one' is the wordplay.
'party formerly' becomes 'lib' (abbreviation for Liberal, uk political party. I am not sure about the 'formerly' bit.).
'one' becomes 'i' (Roman numeral).
'a'+'lib'+'i'='ALIBI'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for alibi that I've seen before include "Timely excuse" , "Making excuse" , "Court evidence showing one was elsewhere" , "Legal defence - was elsewhere" , "Defence that one was somewhere else at the time of a crime" .)