Happy about copper being an element (7)
I believe the answer is:
mercury
'an element' is the definition.
(mercury is a chemical element)
'happy about copper' is the wordplay.
'happy' becomes 'merry'.
'about' means one lot of letters goes inside another.
'copper' becomes 'cu' (chemical symbol).
'merry' placed around 'cu' is 'MERCURY'.
'being' is the link.
(Other definitions for mercury that I've seen before include "Planet - liquid metal" , "Planet next to the sun" , "Metal in thermometer or barometer" , "Messenger in Roman mythology" , "Planet - rising with temperature?" .)