The most common and most likely answer to this crossword clue is the 9 letter word COSTUMERS
I've seen this crossword clue in Independent
C
O
S
T
U
M
E
R
S
Buddy explains!

The straight part
We know that "Dressmakers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dressmakers = costumers.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 COSTUMERS mean?
COSTUMERS noun- A person who designs, makes or supplies theatrical costumes; a costumier.
- A person who wears a costume or takes part in cosplay.
- A costume drama.