Old man, old wall decoration (4)
I believe the answer is:
dado
'wall decoration' is the definition.
(I've seen this in another clue)
'old man old' is the wordplay.
'old man' becomes 'dad' (both can mean one's father).
'old' becomes 'o' (abbreviation).
'dad'+'o'='DADO'
(Other definitions for dado that I've seen before include "Panelled lower part of a wall" , "Low wall panel" , "Lower part of the wall of a room" , "Decorated lower part of an interior wall" , "stretch of wall" .)