Crossword Genius

Ailing guy, with poorly body, always neglecting variable item of medical data (5,5)

Ross

I believe the answer is:

blood group

I'm a little stuck... Click here to teach me more about this clue!

'ailing guy' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Another definition for blood group that I've seen is " Type of red stuff".)

I've seen this clue in The Independent.
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