Corrupt race protected by mad Nero (6)
I believe the answer is:
rotten
'corrupt' is the definition.
(I've seen this before)
'race protected by mad nero' is the wordplay.
'race' becomes 'tt'.
'protected by' indicates putting letters inside.
'mad' indicates anagramming the letters.
'nero' is an anagram of 'roen'.
'tt' placed inside 'roen' is 'ROTTEN'.
(Other definitions for rotten that I've seen before include "Decomposed" , "'Decayed, stinking (6)'" , "Morally despicable" , "Lousy" , "Decayed - to the core" .)