Endlessly sad, relative not quite fitting in sweater (5)
I believe the answer is:
sauna
'endlessly sad relative not quite fitting in sweater' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'relative not quite fitting in sweater' is the wordplay.
'relative' becomes 'aunt' (aunt is a type of relative).
'not quite' means to remove the last letter (most but not all of the word).
'fitting in' indicates putting letters inside.
'sweater' becomes 'sa' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'aunt' with its last letter removed is 'aun'.
'aun' going inside 'sa' is 'SAUNA'.
Can you help me to learn more?
(Other definitions for sauna that I've seen before include "Finnish steam bath" , "A Scandinavian steam bath" , "Finnish-style bath" , "Where one is sweaty" , "Steam-heated room" .)