With one again in the old environment (6)
I believe the answer is:
agreed
'with one' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'again in the old environment' is the wordplay.
I cannot quite see how this works, but
'again' could be 're' (prefix meaning 'again') and 're' is found within the answer.
'old' could be 'aged' (similar in meaning) and 'aged' is present in the leftover letters.
No letters remain.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for agreed that I've seen before include "Acceded" , "Let's do it!" , "Was consistent" , "It's a deal!" , "Consented and concurred" .)