Astronauts left through the nose (5)
I believe the answer is:
nasal
'through the nose' is the definition.
(nasal means relating to the nose)
'astronauts left' is the wordplay.
'astronauts' becomes 'NASA' (National Aeronautics and Space Administration).
'left' becomes 'l' (common abbreviation).
'nasa'+'l'='NASAL'
(Other definitions for nasal that I've seen before include "Beak's" , "Concerning the nose" , "Twangy" , "Of sound, as if it came through the nose" , "Like a twang" .)