Bar bill (3)
I believe the answer is:
tab
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">bar bill</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for tab that I've seen before include "Tally at bar (colloq.)" , "ticket" , "Flap; meal bill" , "Check" , "Marker" .)