Initially appearing lively, outgoing, now extremely solitary (5)
I believe the answer is:
alone
'solitary' is the definition.
('alone' can be a synonym of 'solitary')
'initially appearing lively outgoing now extremely' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'appearing lively outgoing now extremely' is 'ALONE'.
(Other definitions for alone that I've seen before include "Single" , "Isolated from others" , "without partner?" , "Exclusively; solitary" , "Lacking company, solitary" .)