According to mythology, ash and slack covering road up -- even bits of hills (9)

I believe the answer is:

yggdrasil

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'hills' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I can't see?

I cannot understand how the rest of the clue works.

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(Other definitions for yggdrasil that I've seen before include "Ash" , "tree, according to legend" .)

I've seen this clue in The Independent.

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