Is such wood knotty? (4)

I believe the answer is:

hard

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'knotty?' is the definition.
('hard' can be similar in meaning to 'knotty')

I cannot understand how the rest of the clue works.

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(Other definitions for hard that I've seen before include "Solid; tough" , "Solid; difficult" , "Tough - laborious" , "impenetrable" , "Emergency lane" .)

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