Nick's short comment on chapter (5)
I believe the answer is:
notch
'nick's' is the definition.
(both can mean a small cut or impression)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for notch that I've seen before include "Indentation - step up" , "Point or degree in a scale" , "Indentation, nick" , "Dent" , "Small V-shaped cut or indentation" .)