Where you spray Mortein spontaneously? (2,3,3)

I believe the answer is:

on the fly

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'where' is the definition.
Both the answer and definition are adverbs. Maybe you can see a link between them that I can't see?

I don't understand the rest of the clue.

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(Another definition for on the fly that I've seen is " In a hurry - lofty hen (anag)".)

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

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