Liberal involved in first-class defence (5)
I believe the answer is:
alibi
'defence' is the definition.
(alibi is a kind of defence)
'liberal involved in first-class' is the wordplay.
'liberal' becomes 'lib'.
'involved in' indicates putting letters inside.
'first class' becomes 'ai' (A1 - top quality).
'lib' going into 'ai' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Legal term from the Latin 'elsewhere'" , "Proof of absence" , "Excuse, defence" , "Evidence one was elsewhere at time of crime" , "I bail (anag.)" .)