Indentation (5)
I believe the answer is:
notch
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">indentation</span>' is the definition.<br/ ><span class="explanation-format__explanation">(notch is a kind of indentation)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)