The most common and most likely answer to this crossword clue is the 9 letter word GRATICULE
I've seen this crossword clue in Financial Times
G
R
A
T
I
C
U
L
E
Buddy explains!

The straight part
We know that "part of sight" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of sight = graticule.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 GRATICULE mean?
GRATICULE noun- A grid of horizontal and vertical lines.
- (specifically) A reticle.
- (specifically) The network of lines of latitude and longitude that make up a coordinate system such as the one used for the Earth.
- A nearly square or nearly rectangular region created by a graticule.