Act with mistakes when naked (3)
I believe the answer is:
law
'act' is the definition.
(law is a kind of act)
'mistakes when naked' is the wordplay.
'mistakes' becomes 'flaws'.
'when naked' means to look at the middle letters.
The middle letters of 'flaws' are 'LAW'.
'with' acts as a link.
(Other definitions for law that I've seen before include "'Possession is nine points of the ...'" , "Act" , "at 16 down [MAN UNITED]" , "Body of rules of conduct for a society" , "Legislation" .)