Sort of ruler in gaol, rich, corrupt (8)
I believe the answer is:
Here is my best explanation:
'sort of ruler' is the definition.
'gaol rich corrupt' is the subsidiary indication.
'corrupt' is an anagram indicator.
'gaolrich' with letters rearranged gives 'oligarch'.
'in' is the link.
(Other definitions for oligarch that I've seen before include "member of power group", "Member of a small governing group", "Rich Russian with political influence", "Member of small ruling group", "Powerful Russian?", "one of small group at top", "member of ruling minority", "Very rich Russian businessman", "member of small ruling class" and "Member of ruling clique".)