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 wordplay.
'back' says the letters should be written backwards.
'number' becomes 'nine' (nine is a kind of number).
'isa' backwards is '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".)