Breaking bail — one's excuse? (5)
I believe the answer is:
alibi
'excuse?' is the definition.
(excuse given by a suspect in an investigation)
'breaking bail one's' is the wordplay.
'breaking' indicates anagramming the letters.
'one' becomes 'i' (Roman numeral).
'bail' is an anagram of 'alib'.
'alib'+'i'='ALIBI'
(Other definitions for alibi that I've seen before include "Proof of being elsewhere at the time of a crime." , "Evidence one was not at crime scene" , "It shows one wasn't at crime scene" , "Legal proof of innocence" , "Plea of absence" .)