Subject to update from bank? That’s putting it mildly (14)
I believe the answer is:
understatement
'that's putting it mildly' is the definition.
(understating things or putting them mildly)
'subject to update from bank?' is the wordplay.
'subject to' becomes 'under' (being subject to a law is being under it).
'update from bank?' becomes 'statement' (a bank statement).
'under'+'statement'='UNDERSTATEMENT'
(Other definitions for understatement that I've seen before include "Very modest account" , "The downplaying of something" , "Litotes for instance" , "Meiosis or litotes" , "Opposite of exaggeration" .)