It's used for duplicating clients' letters (7)
I believe the answer is:
stencil
'it's used for duplicating' is the definition.
I can't tell whether this defines the answer.
'clients' letters' is the wordplay.
'letters' is an anagram indicator.
'clients' anagrammed gives 'STENCIL'.
Can you help me to learn more?
(Other definitions for stencil that I've seen before include "Create design" , "Cut sheet used for painting design through" , "It helps get uniform print-out" , "means of decoration" , "decoration? One must be cut out for it" .)