Highly skilled (9)
(Other definitions for masterful that I've seen before include "Domineering" , "Dominant" , "overbearing" , "Having dominating quality" , "Authoritative" .)
'highly skilled' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for accomplished that I've seen before include "Talented - performed" , "Highly skilled" , "Highly proficient" , "executed" , "Performed - completed - with a high degree of skill" .)