Defence starts to analyse legal issue behind injunction (5)
I believe the answer is:
alibi
'defence' is the definition.
(alibi is a kind of defence)
'starts to analyse legal issue behind injunction' is the wordplay.
'starts to' indicates taking the first letters.
The initial letters of 'analyse legal issue behind injunction' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Story confirming innocence" , "in order to prove I was elsewhere" , "Reason often given to exculpate someone" , "Legal defence proving innocence" , "Court evidence showing one was elsewhere" .)