Once, once (4)
I believe the answer is:
erst
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">once once</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for erst that I've seen before include "in the past" , "in former times" , "Rest turned formerly" , "At first; formerly" .)