ABOARD

Possibly a container-and-contents indicator indicating that letters or words are to be placed inside SS.

Sample clues:

Food taken aboard in large quantites (6)

Shopping area aboard ship supplies underwear (6)

May be a bits-and-pieces indicator indicating the letters ON.

Sample clues:

Meat a dog has taken aboard (6)

Serviceman getting aboard has to persevere (7,2)

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