Actively working (2,4)

I think it could be either:

Which length description are you looking for?:

2,3,8

2,4

on duty

'actively working' is the definition.
(I've seen this in another clue)

This is the entire clue.

(Other definitions for on duty that I've seen before include "Working at a particular time" , "As working policemen are" , "What working nurses are" , "Engaged in one's regular work" , "(Of soldier) working" .)

I've seen this clue in The Times and the Universal.

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