Legal defence excellent trapping Liberal (5)
I believe the answer is:
alibi
'legal defence' is the definition.
(I've seen this before)
'excellent trapping liberal' is the wordplay.
'excellent' becomes 'ai' (resembles 'A1', old-fashioned term for excellent).
'trapping' means one lot of letters goes inside another.
'liberal' becomes 'lib'.
'ai' going around 'lib' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Answer to charge - was somewhere else" , "Defence used by an accused person" , "Accused person's defence" , "Plea of absence" , "Story confirming innocence" .)