Your writer writes in crimson — not green (7)
I believe the answer is:
ripened
'green' is the definition.
Both the definition and answer are adjectives. Perhaps you can see an association between them that I don't see?
'your writer writes in crimson' is the wordplay.
I cannot really understand how this works, but
'writer' could be 'ed' (I've seen this in other clues) and 'ed' is located in the answer.
The remaining letters 'ripen' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'not' acts as a link.
Can you help me to learn more?
(Other definitions for ripened that I've seen before include "(Of fruit) matured" , "Became mature" , "Matured, aged" .)