A transgression in essence initially stupid (7)
I believe the answer is:
asinine
'stupid' is the definition.
(thesaurus)
'a transgression in essence initially' is the wordplay.
'transgression' becomes 'sin' (sin is a kind of transgression).
'initially' says to take the initial letters.
The first letter of 'essence' is 'e'.
'a'+'sin'+'in'+'e'='ASININE'
(Other definitions for asinine that I've seen before include "Donkey-like" , "Foolish, silly" , "'Mindless, vacuous (7)'" , "Senseless" , "Silly like a donkey" .)