The most common and most likely answer to this crossword clue is the 11 letter word FASHIONISTA
I've seen this crossword clue in The Times
F
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More clues leading to the result FASHIONISTA
Buddy explains!

The straight part
We know that "A dress-conscious trendy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a dress-conscious trendy = fashionista.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 FASHIONISTA mean?
FASHIONISTA noun- A person who creates or promotes high fashion, i.e. a fashion designer or fashion editor.
- A person who dresses according to the trends of fashion, or one who closely follows those trends.