Is a back number and stupid (7)
I believe the answer is:
Here is my best explanation:
'stupid' is the definition.
'is a back number' is the subsidiary indication.
'back' says the letters should be written in reverse.
'number' becomes 'nine' (nine is a kind of number**).
'isa' reversed gives 'asi'.
'and' is the link.
(Other definitions for asinine that I've seen before include "Senseless", "Is inane to be foolish", "Daft", "Foolish like donkey", "Stupid (as a donkey?)", "Jenny's often like this", "senseless", "Being stupid", "Really silly, ridiculous" and "Foolish, silly".)