Dull brown, oddly cheesy top? You'd be a fool to wear it (6,3)
I believe the answer is:
dunces cap
'you'd be a fool to wear it' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'dull brown oddly cheesy top?' is the wordplay.
'dull brown' becomes 'dun' (I've seen this in another clue).
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'top?' becomes 'cap' (both can mean to beat or outdo).
The alternating letters of 'cheesy' are 'ces'.
'dun'+'ces'+'cap'='DUNCES CAP'
Can you help me to learn more?
(Other definitions for dunces cap that I've seen before include "Headgear used to humiliate slow learners" , "Classroom idiot's headgear" , "Ignorant boy's headgear" .)