Ivory trades reported these in blood (5,5)

I believe the answer is:

white cells

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

'these in blood' is the definition.
(blood contains white blood cells)

'ivory trades reported' is the wordplay.
I cannot really understand how this works, but
'ivory' could be 'white' (whitish colour) and 'white' is present in the answer.
The remaining letters 'cells' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Telegraph.

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