A single time (4)
I believe the answer is:
once
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">a single time</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 once that I've seen before include "A single time" , "Just as a single occurrence" , "Some time back" , "One-time musical film and song" , "in bygone era" .)