Treat prince having swallowed ecstasy (4)
I believe the answer is:
heal
'treat' is the definition.
(thesaurus)
'prince having swallowed ecstasy' is the wordplay.
'prince' becomes 'hal' (I've seen this before).
'having swallowed' is an insertion indicator.
'ecstasy' becomes 'e' (informal term for the drug).
'hal' enclosing 'e' is 'HEAL'.
(Other definitions for heal that I've seen before include "Cure, mend" , "Of wound, mend and get better" , "Cure, make well" , "remedy" , "Cure, recover" .)