Pass out leaving very little impression (5)
I believe the answer is:
faint
'pass out' is the definition.
(fainting is a kind of passing out)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for faint that I've seen before include "dull" , "Swoon - pale" , "hard to hear" , "feeble" , "Hard to see" .)