In Italian, "good" can be translated as "buono" or as "bravo".
"Buono" means good as opposed to evil. It's also used for food.
"Bravo" means good as in "skilled" or "talented".
So in this case you need "bravo". If you used "buono", you'd be saying that the writer is a good person. However, just to complicate things, there's an exception: "un brav'uomo" = "un buon uomo" (a good man).
Besides, it would sound more natural to put "bravo" before the noun: un bravo scrittore.
"uno" goes before masculine words begining with z,s +consonant, ps or gn
'clever' was also given as a hint and marked wrong. I would expect 'good writer' to be scrittore buono
I believed that the Italian word "Bravo" is used as an interjection! When a person does something well or completed a project or problem correctly, Italians exclaim, "Bravo!"
I also thought that it was a kind of exclamation so I wrote 'Sei uno scrittore, bravo!' Luckily punctuation doesn't matter here so my answer was accepted, although I was obviously wrong.
You know if you put in a typo when writing this, it would be pretty ironic.