Supplementary part of document belonging to firm (5)
I believe the answer is:
inset
'supplementary part of document' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'belonging to firm' is the wordplay.
'belonging to' becomes 'in' (I've seen this in other clues).
'firm' becomes 'set' (both can mean fixed).
'in'+'set'='INSET'
Can you help me to learn more?
(Other definitions for inset that I've seen before include "day for teachers" , "Something placed into another using tines" , "Piece sewn into a garment" , "Part of magazine" , "subsidiary diagram" .)