Adagio or allegro (5)
I believe the answer is:
tempo
'adagio or allegro' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tempo that I've seen before include "Rhythm or speed as in music" , "Musical beat or rhythm" , "Pace - pacing" , "Pace of a musical piece" , "Speed of 3A" .)