Don to back down … (3,2)
I believe the answer is:
put on
'don' is the definition.
(I know that don can be written as put on)
'back down' is the wordplay.
'back' is a reversal indicator.
'down' becomes 'not up' (I've seen this in another clue).
'notup' reversed gives 'PUT-ON'.
'to' acts as a link.
(Other definitions for put on that I've seen before include "in an unnatural way" , "Increase as with weight" , "Donned" , "Don clothes or stage play" , "Don; mount (a play)" .)