Party member taking part in first-class defence of the accused (5)
I believe the answer is:
alibi
'defence of the accused' is the definition.
'alibi' can be an answer for 'defence' (defence given to police by a suspect). I'm unsure of the 'of the accused' bit.
'party member taking part in first-class' is the wordplay.
'party member' becomes 'lib' (I can't explain this - if you can you should give a lot more credence to this answer).
'taking part in' means one lot of letters goes inside another.
'first class' becomes 'ai' (Al - top quality).
'lib' placed within 'ai' is 'ALIBI'.
Can you help me to learn more?
(Other definitions for alibi that I've seen before include "Legal defence proving innocence" , "Claim put to court" , "Evidence one was not present" , "excuse given" , "Excuse, defence" .)