Fibs 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)
'fibs about drink' is the wordplay.
'fibs' becomes 'lies' (informal term for lies).
'about' means one lot of letters goes inside another.
'drink' becomes 'tot' (a small alcoholic drink).
'lies' placed around 'tot' is 'LITOTES'.
(Other definitions for litotes that I've seen before include "No small task, for example" , "not exactly truthful, for example" , "in which not a little is a lot?" , "Figure of speech about toilets" , "Ironical understatement (grammar)" .)