Understatement (7)
I believe the answer is:
litotes
'understatement' is the definition.
(I've seen this before)
This is all the clue.
(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" .)