Bad melons can be serious! (6)
I believe the answer is:
solemn
'serious' is the definition.
(similar in meaning)
'bad melons' is the wordplay.
'bad' indicates anagramming the letters.
'melons' is an anagram of 'SOLEMN'.
'can be' is the link.
(Other definitions for solemn that I've seen before include "pious" , "Unsmiling" , "Melons (anag.)" , "gloomy" , "'Dignified, sombre (6)'" .)