Stop working for an aristocrat (5)
I believe the answer is:
baron
'for an aristocrat' is the definition.
(baron is a kind of aristocrat)
'stop working' is the wordplay.
'stop' becomes 'bar' (bar can mean to stop or prevent).
'working' becomes 'on' (on can mean working or activated).
'bar'+'on'='BARON'
(Other definitions for baron that I've seen before include "Powerful financier" , "powerful man owning lots of dailies?" , "See nobleman" , "Title of rank" , "An English lord or a magnate" .)