Defence starts to address legal issue behind injunction (5)
I believe the answer is:
alibi
'defence' is the definition.
(alibi is a kind of defence)
'starts to address legal issue behind injunction' is the wordplay.
'starts to' suggests taking the first letters.
The initial letters of 'address legal issue behind injunction' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Proof of being somewhere else at the time" , "Excuse for failure" , "Defence against charge - was seen elsewhere" , "it may help you to get off" , "that gets him off the hook?" .)