Tells tales about infant? That could be an understatement (7)
I believe the answer is:
litotes
'an understatement' is the definition.
(I've seen this before)
'tells tales about infant?' is the wordplay.
'tells tales' becomes 'lies' (I've seen this in another clue).
'about' indicates putting letters inside.
'infant?' becomes 'tot' (tot means a young child).
'lies' going around 'tot' is 'LITOTES'.
'that could be' acts as a link.
(Other definitions for litotes that I've seen before include "Literary use of understatement" , "no overstatement" , "Figure of speech about toilets" , "Rhetorical understatement - toilets (anag)" , "Ironic understatement, like shan't be sorry" .)