Musician's assistant paid for Monroe to jiggle (9)
I believe the answer is:
metronome
'musician's assistant' is the definition.
'paid for monroe to jiggle' is the wordplay.
'paid for' becomes 'met' (eg a rent payment might be met).
'to jiggle' indicates an anagram.
'monroe' with letters rearranged gives 'ronome'.
'met'+'ronome'='METRONOME'
(Other definitions for metronome that I've seen before include "A pacemaker" , "Pendulum to indicate tempo of music" , "Device to help regulate tempo" , "Device that sounds tempo for music practice" , "Musician's time device" .)