Mark as correct (4)
I believe the answer is:
tick
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mark as correct</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 tick that I've seen before include "Blood-sucking arachnid" , "Mark correct, parasite" , "Minute" , "Check-mark or blood-sucking mite" , "Insect; noise of watch" .)