During the Mission "Job One" you need to run to the window in the office and jump out on the ledge within 5 minutes. The timer starts after the sewer explosion and you climb out from the toilet.
It's strongly recommended that you complete the game once and have uppgraded your weapons and character a bit and doing it on easy.
The Tutorial regarding Momentum and the elevator Cinematic doesnt count towards your 5 minutes.
Bringing a good shooting weapons and a few good grenades helps.
You dont need to start from the beginning, you can choose chapter 1 Job One