Casino IOU (4)
I believe the answer is:
chit
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">casino iou</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 chit that I've seen before include "Docket; pert girl" , "Impudent youngster" , "Brief note" , "Official voucher" , "bill to pay" .)