(Musically) smooth (6)
I believe the answer is:
legato
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">musically smooth</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)