New rosette marks outstanding piece of skill (12)
I believe the answer is:
masterstroke
'outstanding piece of skill' is the definition.
(a masterstroke is a display of skill)
'new rosette marks' is the wordplay.
'new' indicates an anagram.
'rosette'+'marks'='rosettemarks'
'rosettemarks' is an anagram of 'MASTERSTROKE'.
(Other definitions for masterstroke that I've seen before include "Brilliant act" , "Clever idea" , "outstanding piece of strategy" .)