Took the attitude that dopes shouldn’t be so silly (5)
I believe the answer is:
posed
'took the attitude that dopes shouldn't' is the definition.
I can't judge whether this definition defines the answer.
'be so silly' is the wordplay.
'be' becomes 'pose' (posing is a kind of being).
'so silly' becomes 'd' (I can't justify this - if you can you should believe this answer much more).
'pose'+'d'='POSED'
Can you help me to learn more?
(Other definitions for posed that I've seen before include "Presented (a problem)" , "Presented (a question)" , "Took a stance" , "Modelled" , "Sat (for artist)" .)