One's destiny as a well-rounded sweetheart (4)
I believe the answer is:
fate
'one's destiny' is the definition.
(I've seen this in another clue)
'rounded sweetheart' is the wordplay.
'rounded' becomes 'fat' (associated in meaning).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'fat'+'e'='FATE'
'as a well' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for fate that I've seen before include "What must be" , "Destiny, kismet" , "Final issue" , "Lot -- doom" , "Predetermined end" .)