Reminder or nudge (4)
I believe the answer is:
prod
I believe this clue is a double definition.
'reminder' is the first definition.
(I've seen this before)
'nudge' is the second definition.
(I know that nudge can be written as prod)
(Other definitions for prod that I've seen before include "Provide stimulation" , "Poke with stiff finger to urge" , "Spur or goad" , "Remind (with elbow or stick?)" , "Poke; reminder" .)