Speed of a musical piece (5)
I believe the answer is:
tempo
'speed of a musical piece' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tempo that I've seen before include "Speed at which piece of music is played" , "Muscial time" , "What a metronome provides" , "Rate and rhythm in music" , "Speed of 3A" .)