One takes turns at the door? (4)
I believe the answer is:
knob
'door?' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'one takes turns at' is the wordplay.
'one' becomes 'kb' (I can't justify this - if you can you should believe this answer much more).
'takes' is an insertion indicator.
'turns' says the letters should be written backwards.
'at' becomes 'on' (location).
'on' back-to-front is 'no'.
'kb' going around 'no' is 'KNOB'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for knob that I've seen before include "Handle or dial" , "Rounded projection" , "Prominent rounded hill" , "Round door handle or lump of butter" , "Ball" .)