The most common and most likely answer to this crossword clue is the 5 letter word ALONE
I've seen this crossword clue in Financial Times
A
L
O
N
E
More clues leading to the result ALONE
- Sailor rejecting seafood, being sole
- Unique beer catches on
- A learner, individual without any classmates?
- Reportedly a sum lent independently
- Initially appearing lively, outgoing, now extremely solitary
- 1 + 50 + 1 = 1?
- Bitter, working in solitary
- Audacious League of Nations enterprise originally unsupported
- Perhaps bitter about performing solo
- Unaccompanied in general on errands
Buddy explains!

The straight part
We know that "solitary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case solitary = alone.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.
What does ALONE mean?
ALONE adjective- By oneself, solitary.
- Apart from, or exclusive of, others.
- Considered separately.
- Without equal.
- Unique; rare; matchless.
- By oneself; apart from, or exclusive of, others; solo.
- Without outside help.
- Exclusively.