Garment, one sort of blue, that may be inside the front door (9)
I believe the answer is:
vestibule
'garment one sort of blue that may be inside the front door' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'garment one sort of blue' is the wordplay.
'garment' becomes 'vest' (vest is a type of garment or item of clothing).
'one' becomes 'i' (Roman numeral).
'sort of' is an anagram indicator.
'blue' anagrammed gives 'bule'.
'vest'+'i'+'bule'='VESTIBULE'
Can you help me to learn more?
(Other definitions for vestibule that I've seen before include "reception" , "room" , "part of house" , "Entrance hall, porch" , "Antechamber, hall" .)