The most common and most likely answer to this crossword clue is the 5 letter word FLUFF
I've seen this crossword clue in The Guardian
F
L
U
F
F
More clues leading to the result FLUFF
Buddy explains!

The straight part
We know that "Slip down/Slip/down" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slip down/slip/down = fluff.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 FLUFF mean?
FLUFF noun- Anything light, soft or fuzzy, especially fur, hair, feathers.
- Anything inconsequential or superficial.
- A lapse or mistake, especially a mistake in an actor's lines.
- Marshmallow creme.
- A passive partner in a lesbian relationship.
- A fart.
- To make something fluffy.
- To become fluffy, puff up.
- To move lightly like fluff.
- (of an actor or announcer) To make a mistake in one's lines.
- To do incorrectly, for example mishit, miskick, miscue etc.
- To fart.
- To arouse (a male pornographic actor) before filming.