Being at fault, by law, mother punished (11)
I believe the answer is:
blameworthy
'being at fault' is the definition.
(thesaurus)
'by law mother punished' is the wordplay.
'punished' indicates an anagram.
'by'+'law'+'mother'='bylawmother'
'bylawmother' is an anagram of 'BLAMEWORTHY'.
(Other definitions for blameworthy that I've seen before include "Liable to be held responsible for a fault" , "Responsible for a wrong" , "Guilty" , "Culpable" .)