Indentation (5)
I believe the answer is:
notch
'indentation' is the definition.
(notch is a kind of indentation)
This is all the clue.
(Other definitions for notch that I've seen before include "Nick made in something as a record" , "Cleft" , "Small V-shaped cut or indentation" , "Mark indented in wood etc" , "Indentation, nick" .)