Barely perceptible amount of time (6)
I believe the answer is:
minute
This clue is a double definition.
'barely perceptible' is the first definition.
(I've seen this before)
'amount of time' is the second definition.
(unit of time)
(Other definitions for minute that I've seen before include "Sixty seconds" , "Infinitesimal" , "Immeasurably small" , "Small; short time" , "not much time" .)