Justification one needed after jumping bail (5)
I believe the answer is:
alibi
'justification' is the definition.
(alibi is a kind of justification)
'one needed after jumping bail' is the wordplay.
'one' becomes 'i' (Roman numeral).
'needed after' means one lot of letters go next to another.
'jumping' indicates an anagram.
'bail' is an anagram of 'alib'.
'i' after 'alib' is 'ALIBI'.
(Other definitions for alibi that I've seen before include "Excuse for not doing something" , "Reason often given to exculpate someone" , "I bail (anag.)" , "Legal defence - was elsewhere" , "Proof of being elsewhere at the time of a crime." .)