Fool of a dog working with chief (6-4)
I believe the answer is:
mutton-head
'fool' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
'a dog working with chief' is the wordplay.
'a dog' becomes 'mutt' (mutt is a kind of dog).
'working' becomes 'on' (on can mean working or activated).
'with chief' becomes 'head' (synonyms. I am not sure about the 'with' bit.).
'mutt'+'on'+'head'='MUTTON-HEAD'
'of' acts as a link.
Can you help me to learn more?
(Another definition for mutton-head that I've seen is " Idiot -- haunted Tom (anag)".)