A measure in operation (5)

I believe the answer is:

afoot

'in operation' is the definition.
('afoot' can mean taking place or in process)

'a measure' is the wordplay.
'measure' becomes 'foot' (foot is a kind of measure).
'a'+'foot'='AFOOT'

(Other definitions for afoot that I've seen before include "About to happen" , "in preparation" , "Astir" , "Brewing" , "Stirring" .)

I've seen this clue in The Globe and Mail.

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