A retiring habit (10)
I believe the answer is:
'a retiring habit' 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?
'retiring habit' is the wordplay.
'retiring' becomes 'night' (I can't explain this - if you can you should give a lot more credence to this answer).
'habit' becomes 'dress' (I've seen this before).
Can you help me to learn more?
(Other definitions for nightdress that I've seen before include "item of bedwear" , "Sleeping attire" .)