Plea by one after bail is arranged (5)
I believe the answer is:
alibi
'plea' is the definition.
(I've seen this before)
'one after bail is arranged' is the wordplay.
'one' becomes 'i' (Roman numeral).
'after' means one lot of letters go next to another.
'is arranged' indicates anagramming the letters.
'bail' anagrammed gives 'alib'.
'i' put after 'alib' is 'ALIBI'.
'by' is the link.
(Other definitions for alibi that I've seen before include "Court evidence showing one was elsewhere" , "Suspect's excuse" , "Pretext; court excuse" , "I bail (anag.)" , "Story confirming innocence" .)