The most common and most likely answer to this crossword clue is the 8 letter word DIAGONAL
I've seen this crossword clue in Independent
D
I
A
G
O
N
A
L
More clues leading to the result DIAGONAL
- Oblique call about endless pain
- Fancy loading packing area on a slope
- Struggle to keep within ring that’s on a slant
- Help returning along wiggly line
- Pet abandoned by poorly painted goal line between corners
- Slanting support raised along breaks
- Cross, having endless pain in face
- Crooked Shakespearean character enters country from the south
- Help over a long wiggly line
- Cross having endless pain in face
Buddy explains!

The straight part
We know that "that’s on a slant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that’s on a slant = diagonal.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 DIAGONAL mean?
DIAGONAL noun- A line joining non-adjacent vertices of a polygon.
- Anything forming or resembling such a line, particularly:
- Joining two nonadjacent vertices (of a polygon or polyhedron).
- Having slanted or oblique lines or markings.
- Having a slanted or oblique direction.
- Of or related to the cater-corner (diagonally opposite) legs of a quadruped, whether the front left and back right or front right and back left.