Half-dozen with nasty allusion that is wicked (10)
I believe the answer is:
villainous
'is wicked' is the definition.
('villainous' can be similar in meaning to 'wicked')
'half-dozen with nasty allusion' is the wordplay.
'half dozen' becomes 'vi' ('vi' can be a synonym of 'half dozen').
'with' is a charade indicator (letters next to each other).
'nasty' indicates anagramming the letters.
'allusion' is an anagram of 'llainous'.
'vi'+'llainous'='VILLAINOUS'
'that' is the link.
(Other definitions for villainous that I've seen before include "very unpleasant" , "criminal" , "really bad" , "dramatically bad" , "Nasty" .)