Bail arranged by one having a reason for innocence (5)
I believe the answer is:
alibi
'reason for innocence' is the definition.
'alibi' can be an answer for 'reason' (I've seen this before). I'm unsure of the 'for innocence' bit.
'bail arranged by one having a' is the wordplay.
I cannot really see how this works, but
an anagram of 'bail' is 'alib' which is present in the answer.
'one' could be 'i' (Roman numeral) and 'i' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for alibi that I've seen before include "Excuse, in law" , "Excuse for not doing something" , "A claim one was elsewhere when crime took place" , "it may help you to get off" , "Defence in court" .)