Formerly, once (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">formerly</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(archaic term for formerly or long ago)</span><br/ ><br/ >'<span class="explanation-format__original">once</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(archaic term for formerly or long ago)</span><br/ ><br/ ></span>
(Other definitions for erst that I've seen before include "At first; formerly" , "in the past" , "in former times" , "Rest turned formerly" .)