One left one's boy outside the room (5)
I believe the answer is:
salon
'room' is the definition.
(salon is a kind of room)
'one left one's boy outside' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'left' becomes 'l' (common abbreviation).
'one's boy' becomes 'son' (I've seen this in another clue).
'outside' is an insertion indicator.
'a'+'l'='al'
'al' placed within 'son' is 'SALON'.
'the' is the link.
(Other definitions for salon that I've seen before include "Loans out as a reception room" , "social event" , "Literary gathering" , "Strange loans for a hairdresser, perhaps" , "Elegant room - for hairdresser?" .)