Revengeful, formally charge four in five with Ecstasy (10)
I believe the answer is:
vindictive
'revengeful' is the definition.
(I know that revengeful can be written as vindictive)
'formally charge four in five with ecstasy' is the wordplay.
'formally charge' becomes 'indict' (I've seen this before).
'four' becomes 'IV' (Roman numerals).
'in' indicates putting letters inside.
'five with' becomes 'v' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ecstasy' becomes 'E' (informal term for the drug).
'indict'+'iv'='indictiv'
'v'+'e'='ve'
'indictiv' placed into 've' is 'VINDICTIVE'.
Can you help me to learn more?
(Other definitions for vindictive that I've seen before include "full of spite" , "out for revenge" , "Revengeful" , "Spiteful" .)