The most common and most likely answer to this crossword clue is the 2 plus 5 plus 3 letter word ON ONE’S TOD
I've seen this crossword clue in The Guardian
O
N
O
N
E
â
€
™
S
T
O
D
Buddy explains!
The straight part
We know that "alone" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case alone = on one’s tod.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.