Becoming extinct (5,3)
I believe the answer is:
dying out
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">becoming extinct</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 dying out that I've seen before include "Becoming extinct" , "Disappearing" .)