Emphatically, the third person taking Ecstasy's being like Hob (7)
I believe the answer is:
oneself
'emphatically the third' is the definition.
I know nothing about this answer so I can't judge whether this works.
'person taking ecstasy's being like hob' is the wordplay.
'person' becomes 'one' (I've seen this in other clues).
'taking' is an insertion indicator.
'ecstasy's' becomes 'es' (informal term for the drug).
'being like hob' becomes 'lf' (I am not sure about this - if you are sure you should believe this answer much more).
'one' enclosing 'es' is 'onese'.
'onese'+'lf'='ONESELF'
Can you help me to learn more?
(Other definitions for oneself that I've seen before include "Introvert's study" , "A certain person" , "individual" , "Personal pronoun" , "Lose fen (anag)" .)