Sort of ruler in gaol, rich, corrupt (8)
I believe the answer is:
oligarch
'sort of ruler' is the definition.
'gaol rich corrupt' is the wordplay.
'corrupt' is an anagram indicator.
'gaol'+'rich'='gaolrich'
'gaolrich' is an anagram of 'OLIGARCH'.
'in' is the link.
(Other definitions for oligarch that I've seen before include "Mega-rich Russian with political influence" , "Member of ruling clique" , "member of small ruling class" , "Powerful Russian?" , "member of power group" .)