Office worker on old rate (5)
I believe the answer is:
tempo
'rate' is the definition.
(musical rhythm or rate)
'office worker on old' is the wordplay.
'office worker' becomes 'temp' (short for temporary worker).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'temp'+'o'='TEMPO'
(Other definitions for tempo that I've seen before include "What a metronome provides" , "Speed or beat, musically" , "Speed at which piece of music is played" , "Rate and rhythm in music" , "Beat (music)" .)