He may even sound somewhat unreliable! (4)
I believe the answer is:
liar
'he may even sound somewhat unreliable' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for liar that I've seen before include "Perjurer" , "man of fiction?" , "Bearer of false witness" , "Fabricator" , "not straight [one]" .)