Contaminate fully-fledged Ecstasy with speed (10)
I believe the answer is:
adulterate
'contaminate' is the definition.
(I've seen this in another clue)
'fully-fledged ecstasy with speed' is the wordplay.
'fully-fledged' becomes 'adult' (I can't justify this - if you can you should give a lot more credence to this answer).
'ecstasy' becomes 'E' (informal term for the drug).
'with' is a charade indicator (letters next to each other).
'speed' becomes 'rate' (both can mean pace).
'adult'+'e'+'rate'='ADULTERATE'
Can you help me to learn more?
(Other definitions for adulterate that I've seen before include "Contaminate" , "Doctor" , "Make impure by addition" , "Spike" , "Make poorer by adding another substance" .)