Place the chicken in a 5-qt. slow cooker. In a small bowl, combine the soup, mushrooms, sherry, butter, tarragon, Worcestershire sauce and garlic powder; pour over chicken. Cover and cook on low for 4-5 hours or until chicken is tender.
Remove chicken and keep warm.
Place the flour in a small saucepan; gradually whisk in cooking liquid until blended. Bring to a boil; cook and stir for 2 minutes or until thickened.