I've never actually eaten in Clarksburg after wine tasting. But the OSM will have food trucks, they have at least one every day, unless by some odd coincidence they both have to cancel. You'll be able to see the schedule on the event section of the website. Also, they don't allow outside food but you can take food and eat it by your car. You just can't take it in and you can't take a bottle of wine out to your car to have with your meal.
The only one I can think of is Husicks Tap House and Dinkys Diner (I have never seen it open but it is like a mini food truck along the river - across the street from Husicks. The food at Husicks is really good and a nice variety of beers. Old Sugar Mill doesn't have restaurants but maybe small bites (cheese boards) and on occasion food trucks. Definitely check the website before you go for food trucks (I have only seen them in special events). My suggestion would be to go Husicks grab some food to go and head to the Sugar Mill and sit in the lawn and enjoy.
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