Short moment (6)
'short moment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for instant that I've seen before include "Jiffy" , "Occurring immediately, like some fast food" , "Short time, of the current month" , "Short time; immediate" , "The blink of an eye" .)
(Other definitions for second that I've seen before include "Small fraction of a day" , "Instant - support" , "awarded the silver medal?" , "Helper in the ring" , "Short-lived" .)