Check, with "in" (4)
I believe the answer is:
rein
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">check with 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 rein that I've seen before include "Harness item" , "Horse's steering strap" , "Animal controller" , "Hold back" , "Check; control" .)