He reviles a blockhead employer (6)
I believe the answer is:
abuser
'he reviles' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a blockhead employer' is the wordplay.
'blockhead' becomes 'b' (head letter of 'block').
'employer' becomes 'user' (someone who employs or uses something).
'a'+'b'+'user'='ABUSER'
Can you help me to learn more?
(Other definitions for abuser that I've seen before include "He takes undue advantage" , "Bully" , "One who reviles or a misuser of drugs" , "one who uses bad language" , "Person who over-indulges in, say, drugs or drink" .)