Many possess what the eider contributed (4)

I believe the answer is:

down

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'the eider contributed' is the definition.
I can't judge whether this defines the answer.

'many possess' is the wordplay.
'many' becomes 'd' (Roman numeral for 500).
'possess' becomes 'own' (synonyms).
'd'+'own'='DOWN'

'what' is the link.

Can you help me to learn more?

(Other definitions for down that I've seen before include "Fuzz" , "Depressed; swallow" , "Out of action; soft feathers" , "Dispirited" , "Sad; feathers" .)

I've seen this clue in the Evening Standard.

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