Master copy of a client's order (7)
I believe the answer is:
stencil
'master copy' is the definition.
'stencil' can be an answer for 'copy' (I've seen this before). I am not certain of the 'master' bit.
'a client's order' is the wordplay.
'order' indicates anagramming the letters.
'clients' is an anagram of 'STENCIL'.
'of' is the link.
Can you help me to learn more?
(Other definitions for stencil that I've seen before include "Clients for copying device" , "Create design" , "Cut-out for painting patterns or shapes" , "It's perforated for ink or paint to pass through" , "Clients (anag)" .)