By law mother might be culpable (11)
I believe the answer is:
blameworthy
'culpable' is the definition.
('blameworthy' can be a synonym of 'culpable')
'by law mother might be' is the wordplay.
'might be' is an anagram indicator.
'by'+'law'+'mother'='bylawmother'
'bylawmother' with letters rearranged gives 'BLAMEWORTHY'.
(Other definitions for blameworthy that I've seen before include "Guilty" , "Liable to be held responsible for a fault" , "Culpable" , "Responsible for a wrong" .)