Happy to gather copper, type of metal (7)
I believe the answer is:
mercury
'type of metal' is the definition.
'happy to gather copper' is the wordplay.
'happy' becomes 'merry'.
'to gather' means one lot of letters goes inside another.
'copper' becomes 'cu' (chemical symbol).
'merry' going around 'cu' is 'MERCURY'.
(Other definitions for mercury that I've seen before include "Metal in thermometer or barometer" , "Planet - liquid metal" , "Lead singer" , "Messenger in Roman mythology" , "First planet from the Sun" .)