The most common and most likely answer to this crossword clue is the 7 letter word WITH-IT
I've seen this crossword clue in The Times
W
I
T
H
-
I
T
Buddy explains!

The straight part
We know that "Fashionable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fashionable = with-it.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 WITH-IT mean?
WITH-IT adjective- Up-to-date on current fashion and trends