The most common and most likely answer to this crossword clue is the 7 letter word UNITARD
I've seen this crossword clue in The Guardian
U
N
I
T
A
R
D
Buddy explains!

The straight part
We know that "costume for dancer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case costume for dancer = unitard.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 UNITARD mean?
UNITARD noun- A skin-tight garment covering the torso and the legs, sometimes the arms and feet.