A transgression in front of emperor is stupid (7)
I believe the answer is:
asinine
'is stupid' is the definition.
(thesaurus)
'a transgression in front of emperor' is the wordplay.
'transgression' becomes 'sin' (sin is a kind of transgression).
'front of' says to take the initial letters.
The first letter of 'emperor' is 'e'.
'a'+'sin'+'in'+'e'='ASININE'
(Other definitions for asinine that I've seen before include "Daft" , "Donkey-like" , "Really silly, ridiculous" , "Stupid (as a donkey?)" , "In the nature of supposedly stupid animal" .)