Iranians finally in American airspace (5)
I believe the answer is:
sinus
'airspace' is the definition.
(I've seen this before)
'iranians finally in american' is the wordplay.
'finally' suggests the final letters.
'american' becomes 'us' (abbreviation for United States).
The final letter of 'iranians' is 's'.
's'+'in'+'us'='SINUS'
(Other definitions for sinus that I've seen before include "Channel between brain and skull" , "Air-filled cavity of the skull" , "Head space" , "chamber" , "Air cavity in the cranial bones" .)