Warning: old man heading for wife is bit of a dog (7)
I believe the answer is:
forepaw
'a dog' is the definition.
(thesaurus)
'warning old man heading for wife is bit of' is the wordplay.
'warning' becomes 'fore' (used to warn of airborne balls on a golf course).
'old man' becomes 'pa' (both can mean someone's father).
'heading for' means one lot of letters go next to another.
'is bit of' says to take the initial letters.
The first letter of 'wife' is 'w'.
'fore'+'pa'+'w'='FOREPAW'
(Other definitions for forepaw that I've seen before include "Animal's front "hand"" , "Part of a cat - FA power (anag)" , "Animal's front foot" , "(Dog's) front foot" .)