Wall decoration is nothing to father! (4)
I believe the answer is:
dado
'wall decoration' is the definition.
(I've seen this in another clue)
'nothing to father' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'father' becomes 'dad' (informal term for father).
'o' put after 'dad' is 'DADO'.
'is' acts as a link.
(Other definitions for dado that I've seen before include "Architectural feature" , "Differently finished lower part of an interior wall" , "Lower section of wall in room" , "Lower part of the wall of a room" , "Lower part of wall decorated differently from rest" .)