Rest will ignore a prompt (8)
I believe the answer is:
reminder
'prompt' is the definition.
(I've seen this in another clue)
'rest will ignore a' is the wordplay.
'rest' becomes 'remainder' (synonyms).
'will ignore' suggests deleting specific letters.
'remainder' with 'a' removed is 'REMINDER'.
(Other definitions for reminder that I've seen before include "Aide-memoire" , "Aide-mmoire" , "Memory refresher" , "note" , "Memo - souvenir" .)