Weighty melons exchanged (6)
I believe the answer is:
solemn
'weighty' is the definition.
(thesaurus)
'melons exchanged' is the wordplay.
'exchanged' indicates anagramming the letters.
'melons' anagrammed gives 'SOLEMN'.
(Other definitions for solemn that I've seen before include "Serious and formal" , "Serious; deeply sincere" , "'Dignified, sombre (6)'" , "Melons (anag.)" , "Formal, ceremonial" .)