Idiotic number like one above (7)
I believe the answer is:
asinine
'idiotic' is the definition.
(both can mean foolish or silly)
'number like one above' is the wordplay.
'number' becomes 'nine' (nine is a number).
'like' becomes 'asi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one above' means one lot of letters go next to another (in a down clue, some letters go above others).
'nine' put after 'asi' is 'ASININE'.
Can you help me to learn more?
(Other definitions for asinine that I've seen before include "Is inane to be foolish" , "Idiotic" , "simple" , "Absurd" , "Daft" .)