Tar in tumble at sea avoiding line (7)

I believe the answer is:

bitumen

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'tar' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for bitumen that I've seen before include "Mixed-up tar" , "Asphalt" , "coverage on the streets?" , "Substance for surfacing roads" , "Tarry substance used to surface roads and roofs" .)

I've seen this clue in The Sun.

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