Musician's timekeeper (9)
I believe the answer is:
metronome
'musician's timekeeper' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for metronome that I've seen before include "Device that sounds tempo for music practice" , "Musical timekeeper" , "A pacemaker" , "Device to help regulate tempo" , "Ticker setting tempo of music" .)