Even Reg is wrong about retribution (7)
I believe the answer is:
revenge
'retribution' is the definition.
(I know that retribution can be written as revenge)
'even reg is wrong about' is the wordplay.
'is wrong' indicates anagramming the letters.
'about' indicates putting letters inside.
'reg' anagrammed gives 'rge'.
'even' placed within 'rge' is 'REVENGE'.
(Other definitions for revenge that I've seen before include "An eye for an eye" , "Satisfaction taken for injury" , "Retribution" , "Ship remembered in verse" , "Malicious retaliation" .)