Collapse inwards (7)
I believe the answer is:
implode
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">collapse inwards</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 implode that I've seen before include "Violently collapse inwards" , "Collapse suddenly inwards" , "Cause to burst inwards" , "Burst inward" .)