He’s trying to drum up business, the dog! (6)
I believe the answer is:
barker
This clue could be a double definition.
'he's trying to drum up business' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'dog' is the second definition.
(I know that barker is a type of dog)
'the' is the link.
Can you help me to learn more?
(Other definitions for barker that I've seen before include "setter, perhaps" , "Loud showman and dog" , "One stands at a carnival giving loud, colourful, sales talk" , "Tout" .)