The most common and most likely answer to this crossword clue is the 4 plus 4 plus 5 letter word HEAD OVER HEELS
I've seen this crossword clue in Independent
H
E
A
D
O
V
E
R
H
E
E
L
S
Buddy explains!
The straight part
We know that "so in love?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case so in love? = head over heels.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.