Customer is right in court (6)
I believe the answer is:
client
'customer' is the definition.
(synonyms)
'right in court' is the wordplay.
'right' becomes 'lien' (legal right).
'in' is an insertion indicator.
'court' becomes 'ct' (abbreviation used in road names).
'lien' inserted within 'ct' is 'CLIENT'.
'is' is the link.
(Other definitions for client that I've seen before include "One employing lawyer" , "Professional person's customer" , "Customer of professional person" , "Customer - dependant" , "Buyer" .)