Salesman's talk about a new dog (7)
I believe the answer is:
spaniel
'dog' is the definition.
(spaniel is a kind of dog)
'salesman's talk about a new' is the wordplay.
'salesman's talk' becomes 'spiel' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'new' becomes 'n' (abbreviation as in New Testament NT).
'a'+'n'='an'
'spiel' placed around 'an' is 'SPANIEL'.
Can you help me to learn more?
(Other definitions for spaniel that I've seen before include "Dog with drooping ears" , "Long-eared dog" , "[HOUND]" , "Type of dog" , "Gundog; obsequious type" .)