Crossword Genius

In a manner, homing pigeons go by it (3)

Ross

I believe the answer is:

air

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'in' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe you can see a link between them that I don't see?

I cannot understand how the rest of the clue works.

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(Other definitions for air that I've seen before include "Short song" , "The earth's atmosphere" , "Broadcast; mien" , "Tune (on the G string?)" , "Space above and around" .)

I've seen this clue in the Evening Standard.
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