Duet playing core of best musical works (6)
I believe the answer is:
etudes
'musical works' is the definition.
(I've seen this in another clue)
'duet playing core of best' is the wordplay.
'playing' indicates an anagram.
'core of' means to look at the middle letters.
The centre of 'best' is 'es'.
'duet' is an anagram of 'etud'.
'etud'+'es'='ETUDES'
(Other definitions for etudes that I've seen before include "Sue Ted over short musical compositions" , "Musical compositions (French)" , "Musical studies" , "Short pieces for solo instruments to show talent" , "Musical exercises (Fr.)" .)