The setter goes before second reminder (4)
I believe the answer is:
memo
'reminder' is the definition.
(I've seen this in another clue)
'the setter goes before second' is the wordplay.
'the setter' becomes 'me' (the person that set the crossword).
'goes before' is a charade indicator (letters next to each other).
'second' becomes 'mo' (both are short periods of time).
'me'+'mo'='MEMO'
(Other definitions for memo that I've seen before include "- Currie, Br. politician" , "In brief, a note of events" , "Short note or message" , "diplomatic communication?" , "A little something to be remembered" .)