Happy planet with no copper? (5)
I believe the answer is:
merry
'happy' is the definition.
(thesaurus)
'planet with no copper?' is the wordplay.
'planet' becomes 'mercury' (Mercury is an example).
'with no' suggests deleting specific letters.
'copper?' becomes 'cu' (chemical symbol).
'mercury' with 'cu' taken away is 'MERRY'.
(Other definitions for merry that I've seen before include "Convivial" , "Drunk - happy" , "Enlivened by drink" , "Jolly and bright" , "Operetta by 11" .)