Russians ultimately in American airspace (5)
I believe the answer is:
sinus
'airspace' is the definition.
(I've seen this before)
'russians ultimately in american' is the wordplay.
'ultimately' suggests the final letters.
'american' becomes 'us' (abbreviation for United States).
The last letter of 'russians' is 's'.
's'+'in'+'us'='SINUS'
(Other definitions for sinus that I've seen before include "chamber" , "Bodily cavity" , "Air-filled cavity in one's head" , "Airspace" , "Cavity in body (connecting to nose)" .)