Customer's legal right in court (6)
I believe the answer is:
client
'customer's' is the definition.
(synonyms)
'legal right in court' is the wordplay.
'legal right' becomes 'lien' (I've seen this before).
'in' is an insertion indicator.
'court' becomes 'ct'.
'lien' inserted inside 'ct' is 'CLIENT'.
(Other definitions for client that I've seen before include "Patron" , "One employing lawyer" , "Computer software" , "Employer of professional" , "Customer for professional service" .)