He reviles a sailor in front of an employer (6)
I believe the answer is:
abuser
'he reviles a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'sailor in front of an employer' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'in front of' means one lot of letters go next to another (some letters go in front of others).
'an employer' becomes 'user' (someone who employs or uses something).
'ab'+'user'='ABUSER'
Can you help me to learn more?
(Other definitions for abuser that I've seen before include "Person who over-indulges in, say, drugs or drink" , "One who reviles or a misuser of drugs" , "He takes undue advantage" , "One mistreating another" , "Bully" .)