Is a back number being stupid (7)
I believe the answer is:
asinine
'being stupid' is the definition.
(I've seen this before)
'is a back number' is the wordplay.
'back' says the letters should be written backwards.
'number' becomes 'nine' (nine is a kind of number).
'is'+'a'='isa'
'isa' back-to-front is 'asi'.
'asi'+'nine'='ASININE'
(Other definitions for asinine that I've seen before include "Donkey-like" , "Jenny's often like this" , "In the nature of supposedly stupid animal" , "Silly like a donkey" , "Is inane to be foolish" .)