As a party member I have a defence (5)
I believe the answer is:
alibi
'a defence' is the definition.
(defence given to police by a suspect)
'as a party member i' is the wordplay.
'as' becomes 'a' (I am not sure about this - if you are sure you should believe this answer much more).
'a party member' becomes 'lib' (short for Liberal).
'a'+'lib'+'i'='ALIBI'
'have' acts as a link.
Can you help me to learn more?
(Other definitions for alibi that I've seen before include "Excuse, defence" , "Court evidence showing one was elsewhere" , "Defence against charge - was seen elsewhere" , "Timely excuse" , "Fact of being elsewhere (legal)" .)