A bold employer is someone who mistreats (6)
I believe the answer is:
abuser
'someone who mistreats' is the definition.
'abuser' can be an answer for 'someone' (abuser is a kind of someone). I am not sure about the rest of the definition.
'a bold employer' is the wordplay.
'bold' becomes 'b' (abbreviation).
'employer' becomes 'user' (someone who employs or uses something).
'a'+'b'+'user'='ABUSER'
'is' acts as a link.
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 uses bad language" , "One mistreating another" , "He takes undue advantage" , "Bully" .)