A dog breeder may be a swindler (4,7,1,3)
I believe the answer is:
sell someone a pup
'swindler' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'a dog breeder may be a' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'one' and 'one' is found in the answer.
'dog' could be 'pup' (I have seen 'Young dog ' mean 'pup' so perhaps 'dog' could also mean 'pup') and 'pup' is located in the answer.
'be a' could be 'a' and 'a' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sell someone a pup that I've seen before include "Pull a fast one" , "getting bread for young boxer?" , "involving lightweight boxer, perhaps" , "Act dishonestly" .)