Father's middle daughter falls for new beau (5)
I believe the answer is:
dandy
'beau' is the definition.
('dandy' can be a synonym of 'beau')
'father's middle daughter falls for new' is the wordplay.
'father' becomes 'da' (I've seen this in other clues).
'middle daughter falls' becomes 'dy' (I can't justify this - if you can you should believe this answer much more).
'for' says to put letters next to each other (I've seen this in other clues).
'new' becomes 'n'.
'dy' after 'n' is 'ndy'.
'da'+'ndy'='DANDY'
Can you help me to learn more?
(Other definitions for dandy that I've seen before include "Fine for American" , "Man about town" , "Old comic" , "comic, 1937-2012" , "Showy dresser" .)