Magistrate’s heartless act that is dishonest (4)
I believe the answer is:
doge
'magistrate's heartless act that is dishonest' is the definition.
'doge' can be an answer for 'magistrate's' (doge is a kind of magistrate). I am not certain of the rest of the definition.
'act that is dishonest' is the wordplay.
'act' becomes 'do' (I've seen this in other clues).
'that is dishonest' becomes 'ge' (I am not sure about this - if you are sure you should believe this answer much more).
'do'+'ge'='DOGE'
(Other definitions for doge that I've seen before include "Former title of the chief magistrate in Venice" , "Old Italian chief magistrate" , "Chief of ancient Venice" , "Magistrate of 14 (historical)" , "Venetian chief magistrate" .)