The master key may be worn (3)
I believe the answer is:
mac
'be worn' is the definition.
I can't tell whether this defines the answer.
'the master key' is the wordplay.
'the master' becomes 'ma' (Master of Arts).
'key' becomes 'c' (musical key).
'ma'+'c'='MAC'
'may' acts as a link.
Can you help me to learn more?
(Other definitions for mac that I've seen before include "one used in shower?" , "A Gaelic son, often in name" , "It may help you keep dry" , "Coat - computer" , "Waterproof raincoat, in short" .)