Being sixteen, worried about Ecstasy (9)
I believe the answer is:
existence
'being' is the definition.
('existence' can be a synonym of 'being')
'sixteen worried about ecstasy' is the wordplay.
'worried' is an anagram indicator.
'about' becomes 'c' (abbreviation for 'circa'**).
'ecstasy' becomes 'e' (informal term for the drug).
'sixteen' is an anagram of 'existen'.
'existen'+'c'+'e'='EXISTENCE'
(Other definitions for existence that I've seen before include "survival" , "Way of life" , "State of being" , "A way of living" , "The business of being" .)