Is a back number and stupid (7)
I believe the answer is:
asinine
'stupid' is the definition.
(thesaurus)
'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' backwards is 'asi'.
'asi'+'nine'='ASININE'
'and' is the link.
(Other definitions for asinine that I've seen before include "Silly like a donkey" , "Senseless" , "Jenny's often like this" , "Daft" , "Idiotic" .)