Iranians at last in American airspace (5)

I believe the answer is:

sinus

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

I can't explain the rest of the clue.

(Other definitions for sinus that I've seen before include "Channel between brain and skull" , "Air-filled cavity in one's head" , "chamber" , "Hollow space" , "Cavity linked to nose" .)

I've seen this clue in The Times and The Sun.

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