Excuse excellent that secures freedom (5)
I believe the answer is:
alibi
'excuse' is the definition.
(alibiing is a kind of excusing)
'excellent that secures freedom' is the wordplay.
'excellent' becomes 'ai'.
'that secures' is an insertion indicator.
'freedom' becomes 'lib' (I can't justify this - if you can you should believe this answer much more).
'ai' going around 'lib' is 'ALIBI'.
Can you help me to learn more?
(Other definitions for alibi that I've seen before include "Courtroom evidence" , "Sort of explanation" , "that gets him off the hook?" , "Defence in court" , "A claim one was elsewhere when crime took place" .)