Jeff's canine associate (4)

I believe the answer is:

mutt

I'm a little stuck... Click here to teach me more about this clue!

This might be a double definition.

'jeff's' is the first definition.
Both the answer and definition are singular nouns.
Maybe there's an association between them I don't understand?

'canine associate' is the second definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

Can you help me to learn more?

(Other definitions for mutt that I've seen before include "Mongrel - fool" , "Chump" , "Setter perhaps" , "Dog of mixed breed" , "Thematic type [DOG]" .)

I've seen this clue in the Evening Standard.

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