Work force in London seen over time (5)

I believe the answer is:

tempo

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'time' is the definition.
(tempo is a kind of time)

I don't understand the rest of the clue.

(Other definitions for tempo that I've seen before include "Speed of 3A" , "Metronome output" , "Cadence" , "Muscial time" , "Speed of a musical piece" .)

I've seen this clue in The Sun.

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