A number divided by six minus ten, stupid (7)
I believe the answer is:
asinine
'stupid' is the definition.
(thesaurus)
'a number divided by six minus ten' is the wordplay.
'number' becomes 'nine' (nine is a kind of number**).
'divided by' means one lot of letters goes inside another.
'minus' indicates named letters should be taken away.
'ten' becomes 'x' ('X' can be a synonym of 'ten').
'six' with 'x' taken out is 'si'.
'a'+'nine'='anine'
'anine' enclosing 'si' is 'ASININE'.
(Other definitions for asinine that I've seen before include "Is Annie? (Anag)" , "simple" , "Foolish, silly" , "Absurd" , "Idiotic" .)