Inability to stand on ones own two feet through being thin? (8)

I believe the answer is:

leanness

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'being thin?' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for leanness that I've seen before include "Thinness" , "thin" , "Lack of fat" .)

I've seen this clue in The Irish News.

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