Fell down; caved in (9)
I believe the answer is:
collapsed
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">fell down</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">caved in</span>' is the second definition.<br/ ><span class="explanation-format__explanation">('collapse' can be a synonym of 'cave in')</span><br/ ><br/ ></span>
(Other definitions for collapsed that I've seen before include "Fell apart" , "Cracked up - gave way" , "laid low" , "Gave way, caved in" .)