Remembering part in AMND: “… a girdle round about the earth”? (9,4)

I believe the answer is:

retaining wall

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But I can't explain it at all! Could you help me understand?

(Other definitions for retaining wall that I've seen before include "not reducing cast of Pyramus and Thisbe?" , "Structure holding back earth or water" , "Barrier holding back earth or water" , "It holds back earth or water" , "Support" .)

I've seen this clue in The Times.

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