Realisation, where I happen to change personality, ultimately (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')
'i happen to change personality ultimately' is the wordplay.
'to change' indicates anagramming the letters.
'ultimately' says to take the final letters.
The last letter of 'personality' is 'y'.
'i'+'happen'='ihappen'
'ihappen' with letters rearranged gives 'epiphan'.
'epiphan'+'y'='EPIPHANY'
'where' is the link.
(Other definitions for epiphany that I've seen before include "revelation results" , "Church festival" , "Festival commemorating the showing of Christ to the Magi" , "A moment of sudden understanding or revelation" , "when the wise men bring gifts" .)