The most common and most likely answer to this crossword clue is the 4 letter word LONE
I've seen this crossword clue in Independent
L
O
N
E
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The straight part
We know that "solo" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case solo = lone.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 LONE mean?
LONE adjective- Solitary; having no companion.
- Isolated or lonely; lacking companionship.
- Sole; being the only one of a type.
- Situated by itself or by oneself, with no neighbours.
- Unfrequented by human beings; solitary.
- Single; unmarried, or in widowhood.