Crossword Genius

Flying by the touchline? (2,3,4)

Ross

I believe the answer is:

on the wing

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'flying' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for on the wing that I've seen before include "Where feathers are" , "down the touchline" , "Where one used to see 7 or 11" , "in flight?" , "Like birds, often" .)

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