The most common and most likely answer to this crossword clue is the 9 letter word HYPEROPIA
I've seen this crossword clue in Financial Times
H
Y
P
E
R
O
P
I
A
Buddy explains!
The straight part
We know that "Sight defect" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sight defect = hyperopia.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 HYPEROPIA mean?
HYPEROPIA noun- A disorder of the vision where the eye focusses images behind the retina instead of on it, so that distant objects can be seen better than near objects.