It opens the door for the boss (4)
I believe the answer is:
knob
This clue is a double definition.
'it opens the door' is the first definition.
(I've seen this in another clue)
'the boss' is the second definition.
(I've seen this before)
'for' is the link.
(Other definitions for knob that I've seen before include "Ball" , "boss" , "Prominent rounded hill" , "Round door-handle" , "Rounded projection" .)