Rest putting article in prompt (9)
I believe the answer is:
remainder
'rest' is the definition.
(I know that rest can be written as remainder)
'putting article in prompt' is the wordplay.
'putting article' becomes 'a' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
'prompt' becomes 'reminder' (I've seen this before).
'a' inserted within 'reminder' is 'REMAINDER'.
Can you help me to learn more?
(Other definitions for remainder that I've seen before include "The part that is left over" , "Surplus" , "Rest - sell off (books) cheap" , "Scrap" , "Balance" .)