(Musically) smooth (6)
I believe the answer is:
legato
'musically smooth' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for legato that I've seen before include "In a smooth and flowing manner (music)" , "Smoothly (mus.)" , "Musically smooth" , "scored smoothly" , "In music, the opposite of staccato" .)