Untruths about drink - that's an understatement (7)
I believe the answer is:
litotes
'that's an understatement' is the definition.
(I've seen this before)
'untruths about drink' is the wordplay.
'untruths' becomes 'lies' (lie is a kind of untruth).
'about' indicates putting letters inside.
'drink' becomes 'tot' (a small alcoholic drink).
'lies' enclosing 'tot' is 'LITOTES'.
(Other definitions for litotes that I've seen before include "No exaggeration" , "Understatement for rhetorical effect" , "Rhetorical understatement - toilets (anag)" , "Town of N. Norway" , "Literary use of understatement" .)