Where to cross a main road airily (7)

I believe the answer is:

flyover

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'where to cross a main road airily' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

This is all the clue.

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(Other definitions for flyover that I've seen before include "part of highway?" , "Raised road" , "The High Road" , "Road or rail bridge" , "Road bridging another" .)

I've seen this clue in the Evening Standard.

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