Keys are not new (4)
I believe the answer is:
aged
'keys are not new' is the definition.
I can't tell whether this defines the answer.
'are not new' is the wordplay.
'are' becomes 'a' (short for 'are', historical unit of measurement).
'not new' becomes 'ged' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'ged'='AGED'
Can you help me to learn more?
(Other definitions for aged that I've seen before include "Became older" , "Old or ancient" , "not a modern copy" , "way over the hill" , "Grew older" .)