Bail arranged by one for the defence (5)
I believe the answer is:
alibi
'the defence' is the definition.
(defence given to police by a suspect)
'bail arranged by one' is the wordplay.
'arranged' is an anagram indicator (letters rearranged).
'by' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'bail' with letters rearranged gives 'alib'.
'alib'+'i'='ALIBI'
'for' is the link.
(Other definitions for alibi that I've seen before include "Legal defence, plea" , "in order to prove I was elsewhere" , "Excuse for failure" , "Sort of explanation" , "Proof of not being at crime site" .)