Keep back alien in deluge (6)
I believe the answer is:
retain
'keep back' is the definition.
('retain' can be a synonym of 'keep back')
'alien in deluge' is the wordplay.
'alien' becomes 'et' (extra terrestrial).
'in' means one lot of letters goes inside another.
'deluge' becomes 'rain' (I've seen this in other clues).
'et' placed inside 'rain' is 'RETAIN'.
(Other definitions for retain that I've seen before include "Keep; remember" , "Not relinquish" , "Hold onto" , "Keep in place" , "'Hold, keep back (6)'" .)