Silly, stupid, bad: a fast scramble (4,2,1,5)
I believe the answer is:
daft as a brush
'silly' is the definition.
'stupid bad a fast scramble' is the wordplay.
'stupid' indicates anagramming the letters (in a stupid order).
'scramble' becomes 'rush' (I can't explain this - if you can you should believe this answer much more).
'bad'+'a'+'fast'='badafast'
'badafast' anagrammed gives 'daftasab'.
'daftasab'+'rush'='DAFT AS A BRUSH'
Can you help me to learn more?
(Other definitions for daft as a brush that I've seen before include "Insane or very stupid (ii)" , "Utterly silly" .)