When ascending, never attempt astronaut's manoeuvre (7)
I believe the answer is:
reentry
'astronaut's manoeuvre' is the definition.
(re-entering the atmosphere)
'when ascending never attempt' is the wordplay.
'when ascending' shows that the letters should be reversed in order (in a down clue, letters go up).
'never' becomes 'ne'er' (synonyms).
'attempt' becomes 'try' (synonyms).
'neer' reversed gives 'reen'.
'reen'+'try'='REENTRY'
(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" .)