Something prompting you to grab a rest (9)

I believe the answer is:

remainder

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'a rest' is the definition.
(I know that rest can be written as remainder)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for remainder that I've seen before include "set of unsold books" , "deposit" , "part that is still to come" , "that's left" , "Rest - sell off (books) cheap" .)

I've seen this clue in The Times.

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