Drifts right in, becomes a permanency (6)

I believe the answer is:

strays

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'drifts' is the definition.
The definition and answer can be both to do with motion as well as being verbs in their -s form.
Maybe there's a link between them I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for strays that I've seen before include "Homeless people's" , "Runaways" , "Wanders off, digresses" , "Wanders off course" , "Wanders off the path, gets lost" .)

I've seen this clue in the Evening Standard.

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