End to regulations in American airspace (5)
I believe the answer is:
sinus
'airspace' is the definition.
(I've seen this before)
'end to regulations in american' is the wordplay.
'end to' indicates one should take the final letters.
'american' becomes 'us' (abbreviation for United States).
The final letter of 'regulations' is 's'.
's'+'in'+'us'='SINUS'
(Other definitions for sinus that I've seen before include "Head space" , "Bodily cavity" , "Anatomical cavity" , "Channel between brain and skull" , "Hole in the head" .)