The most common and most likely answer to this crossword clue is the 10 letter word SLEEVELESS
S
L
E
E
V
E
L
E
S
S
Buddy explains!

The straight part
We know that "summer dress" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case summer dress = sleeveless.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 SLEEVELESS mean?
SLEEVELESS adjective- Of a garment, having no sleeves.
- Wanting a cover, pretext, or palliation; unreasonable; profitless; useless.