Poisonous, half a dozen law books (8)
I believe the answer is:
virulent
'poisonous' is the definition.
(thesaurus)
'half a dozen law books' is the wordplay.
'half a dozen' becomes 'vi' ('vi' can be a synonym of 'half a dozen').
'law' becomes 'rule' (rule is a kind of law).
'books' becomes 'nt' (New Testament).
'vi'+'rule'+'nt'='VIRULENT'
(Other definitions for virulent that I've seen before include "[OPPOSITE OF] Harmless" , "Highly poisonous" , "bitterly hostile" , "Violent or poisonous from a liver nut" , "Highly contagious" .)