Understatement (7)
I believe the answer is:
litotes
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">understatement</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 litotes that I've seen before include "Understatement for rhetorical effect" , "Figure of speech about toilets" , "in which not a little is a lot?" , "Ironical understatement (grammar)" , "Ironic figure of speech" .)