Marsupials must finally settle to sleep (5)

I believe the answer is:

roost

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'sleep' is the definition.
(for birds, to roost is to settle down to sleep)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for roost that I've seen before include "Thin paper" , "Torso (anag)" , "Shetland tidal water" , "Henhouse" , "Bat's resting place" .)

I've seen this clue in The Telegraph.

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