Liberal stopping excellent legal defence (5)
I believe the answer is:
alibi
'legal defence' is the definition.
(I've seen this before)
'liberal stopping excellent' is the wordplay.
'liberal' becomes 'lib'.
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'lib' placed into 'ai' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "it may help you to get off" , "Proof of not being at the scene of the crime" , "Sort of explanation" , "Proof of absence" , "Proof of being elsewhere at the time of a crime." .)