Crackers are not sumptuous (6)
I believe the answer is:
ornate
'sumptuous' is the definition.
(thesaurus)
'crackers are not' is the wordplay.
'crackers' indicates an anagram (informal term for mad).
'are'+'not'='arenot'
'arenot' with letters rearranged gives 'ORNATE'.
(Other definitions for ornate that I've seen before include "Are not (anag.)" , "Richly decorative" , "Over-decorated" , "Rich in decorative detail" , "Richly or elaborately decorated" .)