She stands over space visitor for night cover (5)
I believe the answer is:
sheet
'night cover' is the definition.
(I've seen this in another clue)
'she stands over space visitor' is the wordplay.
'stands' shows that the letters should be reversed in order (I've seen 'standing' mean this (in a down clue, letters stand up)).
'over space visitor' becomes 'te' (I am not sure about this - if you are sure you should believe this answer much more).
'te' back-to-front is 'et'.
'she'+'et'='SHEET'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for sheet that I've seen before include "It usually gets blanket coverage" , "Bed cover" , "Page of postage stamps" , "film" , "Piece of linen, paper, etc" .)