Fast in music (7)
I believe the answer is:
allegro
'fast in music' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for allegro that I've seen before include "Instruction to perform in lively fashion" , "In quick and lively style" , "In rapid tempo" , "(In music) a brisk and lively tempo" , "quickly found in bars?" .)