A worn master key (3)
I believe the answer is:
mac
'a worn master key' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'master key' is the wordplay.
'master' becomes 'ma'.
'key' becomes 'c' (I've seen this in other clues).
'ma'+'c'='MAC'
Can you help me to learn more?
(Other definitions for mac that I've seen before include "Overcoat" , "PC" , "Coat - computer" , "laptop?" , "Raincoat (colloq.)" .)