Pilots finally in US airspace (5)
I believe the answer is:
sinus
'airspace' is the definition.
(I've seen this before)
'pilots finally in us' is the wordplay.
'finally' indicates one should take the final letters.
The last letter of 'pilots' is 's'.
's'+'in'+'us'='SINUS'
(Other definitions for sinus that I've seen before include "snotty enclosure" , "Hollow space" , "Cavity within a bone" , "Passage in the head" , "Nasal cavity" .)