Rotter with Ecstasy and speed (4)
I believe the answer is:
rate
'speed' is the definition.
(rate is a kind of speed)
'rotter with ecstasy' is the wordplay.
'rotter' becomes 'rat' ('rat' can be a synonym of 'rotter').
'with' is a charade indicator (letters next to each other).
'ecstasy' becomes 'e' (informal term for the drug).
'rat'+'e'='RATE'
'and' acts as a link.
(Other definitions for rate that I've seen before include "standard levy" , "Assess the value of" , "Value - scold" , "See 16" , "Pace" .)