Estranged cadet, a 'Neil Armstrong', returned for a bit (9)
I believe the answer is:
alienated
'estranged' is the definition.
(synonyms)
'cadet a neil armstrong returned for a bit' is the wordplay.
'for a bit' says the answer is hidden in the clue.
'returned' says the letters should be written backwards.
'detaneila' is hidden within 'cadet a neil armstrong'.
'detaneila' in reverse letter order is 'ALIENATED'.
(Other definitions for alienated that I've seen before include "Date Liane (anag.)" , "divorced" , "Estranged" , "put off" , "disaffected" .)