A leading organ (4)
I believe the answer is:
nose
This clue might be a double definition.
'a leading' is the first definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'organ' is the second definition.
(nose is a kind of organ)
Can you help me to learn more?
(Other definitions for nose that I've seen before include "Facial part" , "Front of an aircraft" , "Aroma of wine" , "Deserving blame" , "Flair (for wine)" .)