Untidy toilets? That's some understatement! (7)
I believe the answer is:
litotes
'some understatement' is the definition.
(I've seen this before)
'untidy toilets?' is the wordplay.
'untidy' is an anagram indicator.
'toilets' anagrammed gives 'LITOTES'.
'that's' acts as a link.
(Other definitions for litotes that I've seen before include "An ironic understatement about toilets" , "Understatement for rhetorical effect" , "Rhetorical understatement - toilets (anag)" , "No little pain, say" , "No small task, for example" .)