Ruled on corrupt ring (7)
I believe the answer is:
roundel
'ring' is the definition.
(thesaurus)
'ruled on corrupt' is the wordplay.
'corrupt' is an anagram indicator.
'ruled'+'on'='ruledon'
'ruledon' anagrammed gives 'ROUNDEL'.
(Other definitions for roundel that I've seen before include "Circular marker on aircraft" , "Small circular window or design" , "Coloured disc" , "circular device" , "Circular emblem (heraldry)" .)