Assumed to be back in bed (3,2)
I believe the answer is:
put on
'assumed' is the definition.
(synonyms)
'back in bed' is the wordplay.
'back' shows that the letters should be reversed in order.
'in bed' becomes 'not up' (not awake yet).
'notup' reversed gives 'PUT-ON'.
'to be' is the link.
(Other definitions for put on that I've seen before include "Assume < present" , "Don; mount (a play)" , "Apply (to a surface) - present (on stage)" , "Stage -- hoax" , "in an unnatural way" .)