Stupid, a transgression, in essence, ultimately (7)
I believe the answer is:
asinine
'stupid' is the definition.
(thesaurus)
'a transgression in essence ultimately' is the wordplay.
'transgression' becomes 'sin' (sin is a kind of transgression).
'ultimately' suggests the final letters.
The final letter of 'essence' is 'e'.
'a'+'sin'+'in'+'e'='ASININE'
(Other definitions for asinine that I've seen before include "Is inane to be foolish" , "In the nature of supposedly stupid animal" , "Daft" , "Stubborn" , "Really silly, ridiculous" .)