Domino effect (5,8)
I believe the answer is:
chain reaction
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">domino effect</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 chain reaction that I've seen before include "Series of events each caused by the previous one" , "that could get out of control" , "This process should continue" , "Continuing process" , "Series of stimulated responses" .)