Gross mistake (7)
I believe the answer is:
blunder
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">gross mistake</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for blunder that I've seen before include "Stupid mistake" , "A clumsy mistake or movement" , "Clumsy mistake, gaffe" , "Gross mistake" , "A stupid 1 down" .)