Rule internally flawed (3)
I believe the answer is:
law
'rule' is the definition.
(law is a kind of rule)
'internally flawed' is the wordplay.
'internally' indicates the answer is hidden within the clue.
'LAW' can be found hidden inside 'flawed'.
(Other definitions for law that I've seen before include "Collection of rules for a society" , "'Possession is nine points of the ...'" , "Official rule" , "Body of rules of conduct for a society" , "at 16 down [MAN UNITED]" .)