Rate part of system poorly (5)
I believe the answer is:
tempo
'rate' is the definition.
(musical rhythm or rate)
'part of system poorly' is the wordplay.
'part of' says the answer is hidden in the clue.
'TEMPO' is hidden within 'system poorly'.
(Other definitions for tempo that I've seen before include "(Musically) the speed to be played" , "Speed at which a musical composition is to be played" , "Musical time" , "Pace of a musical piece" , "Speed or beat, musically" .)