Crossword Genius

For the arthritic, it's hard (5)

Ross

I believe the answer is:

stiff

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

I cannot understand how the remainder of the clue works.

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(Other definitions for stiff that I've seen before include "Formal" , "Severe - conventional - dead body" , "Firm" , "One dead" , "Not easily bent" .)

I've seen this clue in the Evening Standard.
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