Tip dirt on your smartest clothes? That’s extremely stupid (7)
I believe the answer is:
dumbest
'your smartest clothes? that's extremely stupid' is the definition.
I can't tell whether this defines the answer.
'tip dirt on' is the wordplay.
'tip' becomes 'mbe' (I can't justify this - if you can you should give a lot more credence to this answer).
'dirt' becomes 'dust' (dust is a kind of dirt).
'on' indicates putting letters inside (as in clothing 'on' a person).
'mbe' inserted inside 'dust' is 'DUMBEST'.
Can you help me to learn more?
(Other definitions for dumbest that I've seen before include "Very stupid" , "Thickest" .)