O! Aren't bananas sumptuous? (6)
I believe the answer is:
ornate
'sumptuous?' is the definition.
(thesaurus)
'o aren't bananas' is the wordplay.
'bananas' indicates anagramming the letters (informal term for mad).
'arent' anagrammed gives 'rnate'.
'o'+'rnate'='ORNATE'
(Other definitions for ornate that I've seen before include "Highly embellished" , "Florid" , "Decorated with tan ore" , "Rococo" , "Excessively decorated" .)