When his apartment block is raided by mobsters, an angsty teen discovers that his boring parents were once hit-men of the Yakuza and so he must learn their old ways to defend himself against the violent thugs, who are lead by his estranged Uncle, if he hopes to escape with his life.

I think Nettle's version is an improvement.

>>discovers that his boring parents were once hit-men of the Yakuza

That's? backstory, what the protagonist finds out AFTER the inciting incident. But a? logline focuses on the story looking forward, not on the story looking backwards.

I think Nettle's version is an improvement.

>>discovers that his boring parents were once hit-men of the Yakuza

That's? backstory, what the protagonist finds out AFTER the inciting incident. But a? logline focuses on the story looking forward, not on the story looking backwards.

'When a gang of vengeful mobsters attack their forbidden city (or apartment block), an angsty teenager must stand for his retired mobster-neighborhood as this turns out to be a war between his family people.'

Making the city block or apartment block a forbidden one, where the old hit-men have retired and share a friendly neighborhood can set up your need to avoid 911. Also, the neighborhood must not react with violence as this will create a premise where the audience will feel for these retired people. They will want them to get free of mobsters.

Your logline does not give a great significance to the character's parents so I avoided it in the above version.

The fact that this premise turns out to be a war between his uncle and his parents must not divert the story's attention from the main character (to his family feuds). So make him stand for neighborhood and make his parents back him up with the decision. This will make the premise focus on your main protagonist.

Overall, some tweaks here some tweaks there, you have a decent premise. (Somehow I had a feeling of District 13 movie while reading your version).