Bail out one with a defence (5)
I believe the answer is:
alibi
'a defence' is the definition.
(alibi is a kind of defence)
'bail out one' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'one' becomes 'i' (Roman numeral).
'bail' anagrammed gives 'alib'.
'alib'+'i'='ALIBI'
'with' is the link.
(Other definitions for alibi that I've seen before include "Excuse, in law" , "can offer a valid defence against most charges" , "It shows one wasn't at crime scene" , "Excuse for not doing something" , "Proof of being somewhere else at the time" .)