Rats mistreated by monarch (4)
I believe the answer is:
tsar
'monarch' is the definition.
(tsar is a kind of monarch)
'rats mistreated' is the wordplay.
'mistreated' indicates anagramming the letters.
'rats' is an anagram of 'TSAR'.
'by' acts as a link.
(Other definitions for tsar that I've seen before include "Tyrant" , "Star for a Russian emperor" , "Old Russian ruler" , "Autocrat" , "Male monarch" .)