My mistake! (3,5)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">my mistake</span>' is the definition.<br/ ><span class="explanation-format__explanation">('oops' can be said to acknowledge a mistake)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for oops that I've seen before include "My bad" , "Silly me, I've dropped something!" , "The sound of stupidity" , "Blunder explanation" , "Exclamation on making minor error" .)
(Other definitions for mea culpa that I've seen before include "Acknowledgement of one's guilt (Latin)" , "Old Roman admission of guilt" , "'I am to blame (3,5)'" , "Admission of guilt by the old Romans" , "Sorry!" .)