Naked relative admitting a rule's broken (2,7)
I believe the answer is:
au naturel
'naked' is the definition.
(synonyms)
'relative admitting a rule's broken' is the wordplay.
'relative' becomes 'aunt' (aunt is a kind of relative).
'admitting' means one lot of letters goes inside another.
'broken' indicates anagramming the letters.
'aunt' enclosing 'a' is 'aunat'.
'rule' anagrammed gives 'urel'.
'aunat'+'urel'='AU NATUREL'
(Other definitions for au naturel that I've seen before include "served without dressing" , "Cooked plainly or raw" , "Naked in French, to aural tune" , "Undressed (French)" , "In the nude - in France" .)