For the arthritic, it's hard (5)
I believe the answer is:
stiff
'hard' is the definition.
(similar in meaning)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for stiff that I've seen before include "Formal" , "Severe - conventional - dead body" , "Firm" , "One dead" , "Not easily bent" .)