6 hours is kind of tight but doable. 11:10 might get you on the 11:45 train to Shibuya (arrives around 1pm)...but more likely you'll end up on the 12:20 train getting to Shibuya around 1:30. You'll need to catch the 3:16 back from Shibuya which will get you to Narita at about 4:30. Not a lot of time but you can walk to the main Shibuya intersection from the station in about 10 minutes. Study the maps before you get there so you know where to go. Go for it!