Fall down (8)
I believe the answer is:
collapse
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">fall down</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 collapse that I've seen before include "Fall down or lose strength suddenly" , "Fall down as from weakness" , "Sudden breakdown" , "Slump" , "Fall or disintergrate" .)