Referring to master copy (5)
I believe the answer is:
repro
'copy' is the definition.
(I've seen this in another clue)
'referring to master' is the wordplay.
'referring to' becomes 're' ('re' means regarding or about).
'master' becomes 'pro' (both can mean an expert).
're'+'pro'='REPRO'
(Other definitions for repro that I've seen before include "Modern copy of period style" , "piece of furniture?" , "Imitation of a work of art" , "Roper (anag.)" , "(Of furniture) a copy" .)