Ecstasy developed in happy realisation (8)
I believe the answer is:
epiphany
'realisation' is the definition.
(I have seen 'Sudden realisation' mean 'epiphany' so perhaps 'realisation' could also mean 'epiphany')
'ecstasy developed in happy' is the wordplay.
'ecstasy' becomes 'e' (informal term for the drug).
'developed' is an anagram indicator.
'in'+'happy'='inhappy'
'inhappy' anagrammed gives 'piphany'.
'e'+'piphany'='EPIPHANY'
(Other definitions for epiphany that I've seen before include "A moment of sudden understanding or revelation" , "January 6 feast" , "Christian Festival, showing of Christ to the Magi" , "Moment of sudden revelation (involving three wise men from the east?)" , "Christian feast of showing Christ to Magi" .)