A pork pie specialist (4)
I believe the answer is:
liar
This might be a double definition.
'a pork' is the first definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'pie specialist' is the second definition.
The definition and answer can be both people as well as being singular nouns.
Maybe you can see a link between them that I can't see?
Can you help me to learn more?
(Other definitions for liar that I've seen before include "Person telling stories" , "man of fiction?" , "Teller of untruths" , "not straight [one]" , "Bearer of false witness" .)