Fantastic melons in August (6)
I believe the answer is:
solemn
'august' is the definition.
(thesaurus)
'fantastic melons' is the wordplay.
'fantastic' is an anagram indicator.
'melons' with letters rearranged gives 'SOLEMN'.
'in' acts as a link.
(Other definitions for solemn that I've seen before include "Melons (anag.)" , "Serious; deeply sincere" , "'Earnest, grave (6)'" , "Marked by pomp or ceremony" , "pious" .)