The most common and most likely answer to this crossword clue is the 7 letter word GYMSLIP
I've seen this crossword clue in Independent
G
Y
M
S
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More clues leading to the result GYMSLIP
- Gucci to start with simply formed tunic
- Pinafore dress James picked up by mistake
- Reported James before indiscretion in schoolgirl attire
- At first girl simply removed item of clothing
- Uniform simply resewn by girl in front
- Girl’s first with simply tailored tunic
- Grand escape hides my turning up in school dress
- Girls start with simply designed school tunic
- Good, simply adjusted, schoolgirl’s garment
- Mistake made in fitness class? Not exactly!
Buddy explains!

The straight part
We know that "(school) uniform" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case (school) uniform = gymslip.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 GYMSLIP mean?
GYMSLIP noun- A sleeveless tunic with a pleated skirt, most commonly seen as part of a girl's school uniform.