Customer’s legal right to be in court (6)
I believe the answer is:
client
'customer's' is the definition.
('client' can be a synonym of 'customer')
'legal right to be in court' is the wordplay.
'legal right' becomes 'lien' (I've seen this before).
'to be in' is an insertion indicator.
'court' becomes 'ct'.
'lien' inserted into '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" .)