A party member - one providing excuse to avoid blame (5)
I believe the answer is:
alibi
'excuse to avoid blame' is the definition.
(I've seen this in another clue)
'a party member one' is the wordplay.
'party member' becomes 'lib' (short for Liberal).
'one' becomes 'i' (Roman numeral).
'a'+'lib'+'i'='ALIBI'
'providing' acts as a link.
(Other definitions for alibi that I've seen before include "It shows one wasn't at crime scene" , "Excuse for not doing something" , "Plea of absence" , "Defence of being elsewhere at time of crime" , "Legal defence proving innocence" .)