For all to see in message, father's blunder (4,3)
I believe the answer is:
'blunder' is the definition.
(I've seen this before)
'for all to see in message father's' is the wordplay.
'for all to see' becomes 'u' (universal dvd certificate).
'in' is an insertion indicator.
'message' becomes 'fax'.
'father's' becomes 'pa's' (pa can mean dad or father).
'u' inserted within 'fax' is 'faux'.
(Other definitions for faux pas that I've seen before include "Social indiscretion", "Mistake", "Embarrassing blunder", "Socially awkward act", "Social error".)