Couple upside-down in boudoir (3)
I believe the answer is:
duo
'couple' is the definition.
('duo' can be a synonym of 'couple')
'upside-down in boudoir' is the wordplay.
'upside down' says the letters should be written backwards.
'in' indicates a hidden word.
'oud' can be found hidden inside 'boudoir'.
'oud' backwards is 'DUO'.
(Other definitions for duo that I've seen before include "A team of two" , "Musical work for two" , "Pair (of people, things)" , "Twosome" , "Couple, pair" .)