Mild sarcasm (5)
I believe the answer is:
irony
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">mild sarcasm</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for irony that I've seen before include "Satire, sarcasm" , "the reverse of what you'd expect" , "Mild sarcasm" , "like a metallic element" , "The use of words which is the opposite of what they say" .)