Ancient key set in a precious metal (3-3)
I believe the answer is:
age-old
'ancient' is the definition.
(I've seen this before)
'key set in a precious metal' is the wordplay.
'key' becomes 'e' (a musical key).
'set in' indicates putting letters inside.
'precious metal' becomes 'gold' (gold is a precious metal).
'a'+'gold'='agold'
'e' put within 'agold' is 'AGE-OLD'.
(Other definitions for age-old that I've seen before include "Belonging to times long ago" , "Hallowed by antiquity" , "Having existed for a very long time" , "Lasting from times long ago" , "Extremely ancient" .)