Small flap (3)
I believe the answer is:
tab
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">small flap</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 "Check" , "Tally of orders in bar" , "Small flap - bill" , "Till bill" , "Kind of label for bat" .)