Customer's right defended by court (6)
I believe the answer is:
client
'customer's' is the definition.
('client' can be a synonym of 'customer')
'right defended by court' is the wordplay.
'right' becomes 'lien' (I've seen this before).
'defended by' means one lot of letters goes inside another.
'court' becomes 'ct'.
'lien' put within 'ct' is 'CLIENT'.
(Other definitions for client that I've seen before include "One employing lawyer" , "Buyer" , "Person using professional services" , "(Barrister's, social services') customer" , "Person represented by lawyer" .)