One refusal covers idiot's vacuity (7)
I believe the answer is:
inanity
'vacuity' is the definition.
('inanity' can be a synonym of 'vacuity')
'one refusal covers idiot's' is the wordplay.
'one' becomes 'i' (Roman numeral).
'refusal' becomes 'nay' (I've seen this before).
'covers' indicates putting letters inside.
'idiot' becomes 'nit' (I've seen this before).
'nay' placed around 'nit' is 'nanity'.
'i'+'nanity'='INANITY'
(Other definitions for inanity that I've seen before include "Puerility" , "Silliness in a tiny way" , "Being silly" , "Foolishness" , "act of stupidity" .)