Accompanied by old defendant (7)
I believe the answer is:
accused
'defendant' is the definition.
(synonyms)
'accompanied by old' is the wordplay.
'accompanied' becomes 'acc' (abbreviation).
'by' means one lot of letters go next to another.
'old' becomes 'used' (as in used or second-hand items).
'acc'+'used'='ACCUSED'
(Other definitions for accused that I've seen before include "Indicted" , "Charged with an offence" , "Blamed" , "He who stands in the dock" , "Defendant appearing on a criminal charge" .)