The most common and most likely answer to this crossword clue is the 11 letter word CROSS-HATCH
I've seen this crossword clue in The Guardian
C
R
O
S
S
-
H
A
T
C
H
Buddy explains!
The straight part
We know that "shading" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case shading = cross-hatch.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 CROSS-HATCH mean?
CROSS-HATCH noun- A pattern of crossing lines.
- To mark or fill with a crosshatch pattern.