Fancy us to be stupid (6)
I believe the answer is:
obtuse
'stupid' is the definition.
(similar in meaning)
'fancy us to be' is the wordplay.
'fancy' indicates anagramming the letters.
'us'+'to'+'be'='ustobe'
'ustobe' with letters rearranged gives 'OBTUSE'.
(Other definitions for obtuse that I've seen before include "Dull or blunt" , "Not very perceptive or sensitive" , "boneheaded?" , "Slow-witted; type of angle" , "Blunt - stupid - insensitive" .)