Stupid offence, one committed by an employee at the start (7)
I believe the answer is:
asinine
'stupid' is the definition.
(thesaurus)
'offence one committed by an employee at the start' is the wordplay.
'offence' becomes 'sin' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'committed by' is an insertion indicator (commit can mean imprison).
'at the start' suggests taking the first letters.
The first letter of 'employee' is 'e'.
'sin'+'i'='sini'
'sini' put inside 'an' is 'asinin'.
'asinin'+'e'='ASININE'
(Other definitions for asinine that I've seen before include "In the nature of supposedly stupid animal" , "simple" , "Silly like a donkey" , "Daft" , "Stupid (as a donkey?)" .)