Happy to admit copper is a metal (7)
I believe the answer is:
mercury
'metal' is the definition.
(I know that mercury is a metal)
'happy to admit copper' is the wordplay.
'happy' becomes 'merry'.
'to admit' is an insertion indicator.
'copper' becomes 'Cu' (Cu is the chemical symbol for copper).
'merry' going around 'cu' is 'MERCURY'.
'is a' acts as a link.
(Other definitions for mercury that I've seen before include "Planet next to the sun" , "Messenger of the Gods, or a chemical element" , "First planet from the Sun" , "Atomic number 80" , "Planet - liquid metal" .)