On application, it lets astronauts come home (7)
I believe the answer is:
reentry
'it lets astronauts come home' is the definition.
The definition suggests a singular noun which matches the answer.
'on application' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'application' becomes 'entry' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'entry'='REENTRY'
Can you help me to learn more?
(Other definitions for reentry that I've seen before include "of concern to NASA?" , "final part of space flight" , "Return to the Earth's atmosphere" .)