On a single occasion (4)
I believe the answer is:
once
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">on a single occasion</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 once that I've seen before include "In days gone by" , "Quick examination" , "Cone (anag.)" , "At some time past" , "One-time - one time" .)