The most common and most likely answer to this crossword clue is the 7 letter word COOLEST
I've seen this crossword clue in The Daily Telegraph
C
O
O
L
E
S
T
Buddy explains!
The straight part
We know that "Most fashionable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most fashionable = coolest.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 COOLEST mean?
COOLEST adjective- Having a slightly low temperature; mildly or pleasantly cold.
- Allowing or suggesting heat relief.
- Of a color, in the range of violet to green.
- Of a person, not showing emotion; calm and in control of oneself.
- Unenthusiastic, lukewarm, skeptical.
- Calmly audacious.
- Applied facetiously to a sum of money, commonly as if to give emphasis to the largeness of the amount.
- Of a person, knowing what to do and how to behave; considered popular by others.
- In fashion, part of or fitting the in crowd; originally hipster slang.
- Of an action, all right; acceptable; that does not present a problem.
- Of a person, not upset by circumstances that might ordinarily be upsetting.
- Quietly impudent, defiant, or selfish; deliberately presuming: said of persons and acts.