Its downfall may help those in retirement (5)

I believe the answer is:

eider

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

'retirement' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for eider that I've seen before include "Type of duck" , "Duck supplying down" , "Duck valued for its fine soft down" , "Duck with soft feathers" , "Source of fine soft down" .)

I've seen this clue in The Globe and Mail.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.