Customer's legal right to be in court (6)
I believe the answer is:
client
'customer's' is the definition.
(synonyms)
'legal right to be in court' is the wordplay.
'legal right' becomes 'lien' (type of property right).
'to be in' means one lot of letters goes inside another.
'court' becomes 'ct' (abbreviation used in road names).
'lien' inserted inside 'ct' is 'CLIENT'.
(Other definitions for client that I've seen before include "Computer software" , "Eg, lawyer's customer" , "Person using professional services" , "Person represented by lawyer" , "Customer for professional service" .)