Second seller is a really bad one! (7)
I believe the answer is:
shocker
This clue could be a double definition.
'second seller' is the first definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'really bad one' is the second definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'is a' is the link.
Can you help me to learn more?
(Other definitions for shocker that I've seen before include "Something unsettlingly surprising" , "unpleasant person" .)