I was one of the runners-up for last xword. Looked back at clue for 1 Down, and I have no idea why the answer was SHELL either. One of life's mysteries, folks
Chambers also gives "dare" as "a contrivance with mirrors such as was once used to fascinate larks." Why anyone would want to fascinate a lark, I don't know.
Shell had a popular advertising campaign for some 20yrs. Before my time. That's Shell - that was! With various cartoons suggesting a car had just sped by.