Green energy company operated illegally retaining six employees regularly at fault (13)
I believe the answer is:
environmental
'green' is the definition.
(I've seen this before)
'six employees regularly at fault' is the wordplay.
'six' becomes 'environ' (I can't justify this - if you can you should give a lot more credence to this answer).
'employees' becomes 'men' (men can mean staff).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'at'+'fault'='atfault'
The alternate letters of 'atfault' are 'tal'.
'environ'+'men'+'tal'='ENVIRONMENTAL'
'energy company operated illegally retaining' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for environmental that I've seen before include "Pertaining to the surrounding world" , "To do with one's surroundings" , "Green" .)