Assumed to be standing down (3,2)
I believe the answer is:
put on
'assumed' is the definition.
(synonyms)
'standing down' is the wordplay.
'standing' shows that the letters should be reversed in order (in a down clue, letters stand up).
'down' becomes 'not up' (I've seen this in another clue).
'notup' backwards is 'PUT-ON'.
'to be' is the link.
(Other definitions for put on that I've seen before include "in an unnatural way" , "Affixed" , "Stage -- hoax" , "Don clothes or stage play" , "Don; mount (a play)" .)