Wicked American back, not dead (5)
I believe the answer is:
alive
'not dead' is the definition.
('alive' is the opposite to 'dead')
'wicked american back' is the wordplay.
'wicked' becomes 'evil' (synonyms).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'back' shows that the letters should be reversed in order.
'evil'+'a'='evila'
'evila' back-to-front is 'ALIVE'.
(Other definitions for alive that I've seen before include "Animated and kicking" , "Teeming (with)" , "Breathing, existing" , "Spirited" , "Quick - breathing" .)