The most common and most likely answer to this crossword clue is the 6 letter word GLACIS
I've seen this crossword clue in The Times
G
L
A
C
I
S
Buddy explains!

The straight part
We know that "slope" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slope = glacis.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 GLACIS mean?
GLACIS noun- A gentle incline in front of a fortification.
- The angled armour plate on the front of a tank; glacis plate.
- An erosional or depositional landform, with little slope.
- (postal service) A device for sorting mail which slides parcels across a sloped surface.