Spiteful half-dozen are ruined (8)
I believe the answer is:
viperish
'spiteful' is the definition.
(I've seen this before)
'half-dozen are ruined' is the wordplay.
'half-dozen' becomes 'VI' (Roman numerals for six).
'are ruined' becomes 'perish' (I can't justify this - if you can you should believe this answer much more).
'vi'+'perish'='VIPERISH'
Can you help me to learn more?
(Other definitions for viperish that I've seen before include "like a snake" , "like a traitor?" , "Spiteful" , "Malicious" , "Hostile" .)