They can swim, dear (5)

I believe the answer is:

ducks

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'they' is the definition.
Both the definition and answer are plural nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the rest of the clue.

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(Other definitions for ducks that I've seen before include "Lowers head suddenly" , "Swimming birds with scores of nothing?" , "Evades" , "More than one aquatic bird" , "Teal, wigeon etc" .)

I've seen this clue in the Evening Standard.

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