Her master's about to change a coin (6)
I believe the answer is:
monica
'her' is the definition.
'master's about to change a coin' is the wordplay.
'master' becomes 'ma'.
'about' indicates putting letters inside.
'to change' indicates anagramming the letters.
'coin' is an anagram of 'onic'.
'ma' placed around 'onic' is 'MONICA'.
(Other definitions for monica that I've seen before include "Girl" , "Nice silky woman (anag) -- White House intern" , "Girl's name; am icon (anag.)" , "Womans name" , "Girl's name (sounds like name)" .)