Something prompting you to grab a rest (9)
I believe the answer is:
remainder
'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" .)