"Why did I do that?!" (3)
I believe the answer is:
doh
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">why did i do that?</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for doh that I've seen before include "Homeric grunt of annoyance" , "Stupid me!" , "How foolish of me" , "acknowledgment of stupidity" , "First of the series at 17 Across" .)