This is another straightforward Raider hunt for the Minutemen. After completing Taking Point: Starlight Drive-In, Preston will assign you this new quest. Your first destination is Oberland Station. You can reach this by fast travelling to Lexington, then heading southwest, crossing a bridge along the way.
The Settlers at Oberland Station will ask you to clear Back Street Apparel of its Raiders.
Once at Back Street Apparel, expect resistance both inside and outside the store. The Raiders inside are especially dangerous, given their use of explosives. After you defeat them, return to Oberland Station to give a report, then go back to Sanctuary to update Preston.