The most common and most likely answer to this crossword clue is the 9 letter word LONELIEST
I've seen this crossword clue in Independent
L
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The straight part
We know that "Extremely solitary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case extremely solitary = loneliest.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 LONELIEST mean?
LONELIEST adjective- Unhappy because of feeling isolated from contact with other people.
- (of a place or time) Unfrequented by people; desolate.
- (of a person) Without companions; solitary.