Stupid as yours truly: and square! (7)
I believe the answer is:
asinine
'stupid' is the definition.
(thesaurus)
'as yours truly and square' is the wordplay.
'yours truly' becomes 'i'.
'and' says to put letters next to each other.
'square' becomes 'nine' (I've seen this before).
'as'+'i'+'nine'='ASININE'
(Other definitions for asinine that I've seen before include "Absurd" , "Foolish like donkey" , "Donkey-like" , "Is inane to be foolish" , "Jenny's often like this" .)