Alexander's dad had a sweetheart (4)
I believe the answer is:
pope
'alexander's' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'dad had a sweetheart' is the wordplay.
'dad' becomes 'pop' (synonyms).
'had' means one lot of letters go next to another.
'a sweetheart' becomes 'e' (the heart/centre of 'sweet').
'pop'+'e'='POPE'
Can you help me to learn more?
(Other definitions for pope that I've seen before include "Francis < Alexander" , "Well-versed man" , "man of great authority" , "Head of Roman Catholic Church" , "Holy man" .)