Impulse to be back in, loitering with intent (5)

I believe the answer is:

nisus

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

I don't understand how the remainder of the clue works.

(Other definitions for nisus that I've seen before include "Effort" , "Striving towards a goal" .)

I've seen this clue in the Sydney Morning Herald.

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