One may be missed in an instant (9)
I believe the answer is:
heartbeat
This clue is a double definition.
'one may be missed' is the first definition.
(as in a heart skipping a beat)
'an instant' is the second definition.
(I know that instant can be written as heartbeat)
'in' acts as a link.
(Other definitions for heartbeat that I've seen before include "Circulation noise" , "Pulse of cardiac origin" , "Essential pulsation" , "Animating force" .)