Crossword Genius

Insect necessarily airborne? (3)

Ross

I believe the answer is:

fly

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'insect' is the definition.
(fly is a kind of insect)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for fly that I've seen before include "Run away; smart (colloq.)" , "Cunning; insect" , "One invited into parlour" , "knowing about > trap" , "Move quickly; knowing" .)

I've seen this clue in The Sun.
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