Unit of note with a lower tone (4)
I believe the answer is:
flat
'unit' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'note with a lower tone' is the wordplay.
'note' becomes 'f' (musical note).
'with' is an insertion indicator.
'lower tone' becomes 'lt' (this could be a standard abbreviation I've not previously seen).
'a' placed inside 'lt' is 'lat'.
'f'+'lat'='FLAT'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for flat that I've seen before include "Boring - apartment" , "Even - somewhere to live" , "character on the staff" , "No longer charged" , "Punctured tyre" .)