Couple retreating inside boudoir (3)
I believe the answer is:
duo
'couple' is the definition.
('duo' can be a synonym of 'couple')
'retreating inside boudoir' is the wordplay.
'retreating' is a reversal indicator.
'inside' indicates the answer is hidden within the clue.
'oud' is hidden within 'boudoir'.
'oud' reversed gives 'DUO'.
(Other definitions for duo that I've seen before include "Performing couple" , "Pair (of people, things)" , "entertainers such as Eric and Ernie" , "Couple, pair" , "Pair of performers" .)