Happy to include copper as a member of the periodic table (7)
I believe the answer is:
mercury
'a member of the periodic table' is the definition.
I can't tell whether this defines the answer.
'happy to include copper' is the wordplay.
'happy' becomes 'merry'.
'to include' is an insertion indicator.
'copper' becomes 'cu' (chemical symbol).
'merry' going around 'cu' is 'MERCURY'.
'as' is the link.
Can you help me to learn more?
(Other definitions for mercury that I've seen before include "Messenger of the Gods, or a chemical element" , "Planet nearest the sun" , "Quicksilver" , "Atomic number 80" , "Planet - rising with temperature?" .)