No material covers round stuff, oddly (7)
I believe the answer is:
refusal
'no' is the definition.
(refusing is saying no)
'material covers round stuff oddly' is the wordplay.
'material' becomes 'real' (associated in meaning).
'covers' means one lot of letters goes inside another.
'round' indicates anagramming the letters.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'stuff' are 'suf'.
'suf' anagrammed gives 'fus'.
'real' going around 'fus' is 'REFUSAL'.
(Other definitions for refusal that I've seen before include "Act of a horse stopping short at a jump" , "Dissent" , "Horse's action in declining to jump a fence" , "Negation" , "Seal fur (anag.)" .)