The most common and most likely answer to this crossword clue is the 8 letter word DECREASE
I've seen this crossword clue in The Guardian
D
E
C
R
E
A
S
E
Buddy explains!
The straight part
We know that "Slim" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case slim = decrease.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 DECREASE mean?
DECREASE noun- An amount by which a quantity is decreased.
- A reduction in the number of stitches, usually accomplished by suspending the stitch to be decreased from another existing stitch or by knitting it together with another stitch. See Decrease (knitting).
- Of a quantity, to become smaller.
- To make (a quantity) smaller.