Customer's legal right accepted by court (6)
I believe the answer is:
client
'customer's' is the definition.
('client' can be a synonym of 'customer')
'legal right accepted by court' is the wordplay.
'legal right' becomes 'lien' (I've seen this before).
'accepted by' 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" , "(Barrister's, social services') customer" , "Customer for professional service" , "Eg, lawyer's customer" , "person lawyer represents" .)