Customer's right established in court (6)
I believe the answer is:
client
'customer's' is the definition.
('client' can be a synonym of 'customer')
'right established in court' is the wordplay.
'right' becomes 'lien' (I've seen this before).
'established in' means one lot of letters goes inside another.
'court' becomes 'ct'.
'lien' going within 'ct' is 'CLIENT'.
(Other definitions for client that I've seen before include "Professional person's customer" , "Patron" , "Customer for professional service" , "One employing lawyer" , "Computer software" .)