The most common and most likely answer to this crossword clue is the 6 plus 6 letter word BROKEN HEARTS
I've seen this crossword clue in The Daily Telegraph
B
R
O
K
E
N
H
E
A
R
T
S
More clues leading to the result BROKEN HEARTS
Buddy explains!
The straight part
We know that "What Lothario leaves behind" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what lothario leaves behind = broken hearts.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.