Step in (5)
I believe the answer is:
enter
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">step in</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for enter that I've seen before include "Register formally as a participant" , "Enroll, sign up" , "Record - stage direction - come in" , "Infiltrate" , "Treen (anag.)" .)