381cm seals? (8)

I believe the answer is:

clinches

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'seals?' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Perhaps there's a link between them I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for clinches that I've seen before include "Tight or amorous embraces, in the boxing ring" , "Tight grips in boxing" , "finally settles" .)

I've seen this clue in the Sydney Morning Herald.

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