Musical piece or movement in slow time (6)
I believe the answer is:
adagio
This clue is a double definition.
'musical piece' is the first definition.
'adagio' can be an answer for 'piece' (adagio is a kind of piece). I also know that 'musical' relates to this answer.
'movement in slow time' is the second definition.
(I've seen this before)
(Other definitions for adagio that I've seen before include "Slowly (mus.)" , "'Slowly and gracefully, in music (6)'" , "leisurely movement" , "In music, leisurely" , "Slow piece of music" .)