Iranians at last in American airspace (5)
I believe the answer is:
sinus
'airspace' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(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" .)