Difficult in a way said to be negative? (6)

I believe the answer is:

knotty

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'difficult' is the definition.
(similar in meaning)

I can't explain the rest of the clue.

(Other definitions for knotty that I've seen before include "Tree trunk with hard masses - it's problematic" , "Difficult (like wood hard to chop through?)" , "Complex - hard to solve" , "Awkward" , "Pinewood can be" .)

I've seen this clue in the Evening Standard.

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