Allegro or adagio (5)
I believe the answer is:
tempo
'allegro or adagio' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tempo that I've seen before include "Rhythm, metre" , "What a metronome provides" , "Beat (music)" , "Rhythm of piece of music" , "Rate and rhythm in music" .)