Responsible for one in laboratory initially losing Ecstasy (6)
I believe the answer is:
liable
'responsible for' is the definition.
(thesaurus)
'one in laboratory initially losing ecstasy' is the wordplay.
'one' becomes 'i' (Roman numeral**).
'in' is an insertion indicator.
'laboratory' becomes 'lab' ('lab' can be a synonym of 'laboratory').
'initially' indicates taking the first letters.
The initial letters of 'losing ecstasy' is 'le'.
'i' put into 'lab' is 'liab'.
'liab'+'le'='LIABLE'
(Other definitions for liable that I've seen before include "Responsible at law" , "Exposed to risk" , "'Responsible, answerable (6)'" , "Likely" , "To blame" .)