American lecturer regularly taking ecstasy by herself (5)
I believe the answer is:
alone
'by herself' is the definition.
'american lecturer regularly taking ecstasy' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'lecturer' becomes 'l' (abbreviation for lecturer).
'regularly taking' becomes 'on' (as in 'on drugs').
'ecstasy' becomes 'E' (informal term for the drug).
'a'+'l'+'on'+'e'='ALONE'
(Other definitions for alone that I've seen before include "On one's own" , "Note of charges" , "Solo" , "unique" , "Isolated; exclusively" .)