Venomous six of the Romans die (8)
I believe the answer is:
viperish
'venomous' is the definition.
(thesaurus)
'six of the romans die' is the wordplay.
'six of the romans' becomes 'vi' (six in Roman numerals).
'die' becomes 'perish' (synonyms).
'vi'+'perish'='VIPERISH'
(Other definitions for viperish that I've seen before include "Spiteful" , "Hostile" , "like a snake" , "like a traitor?" , "Malicious" .)