Power of three copper keys (4)
I believe the answer is:
cube
'power of three' is the definition.
(I've seen this before)
'copper keys' is the wordplay.
'copper' becomes 'cu' (Cu is the chemical symbol for copper).
'keys' becomes 'be' (I can't justify this - if you can you should give a lot more credence to this answer).
'cu'+'be'='CUBE'
Can you help me to learn more?
(Other definitions for cube that I've seen before include "Form of chiller" , "Lump of sugar" , "Square block" , "Regular 3-D shape" , "Raise to the third power" .)