The most common and most likely answer to this crossword clue is the 9 plus 4 letter word CARTRIDGE BELT
I've seen this crossword clue in The Times
C
A
R
T
R
I
D
G
E
B
E
L
T
More clues leading to the result CARTRIDGE BELT
Buddy explains!

The straight part
We know that "where slugs collected?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where slugs collected? = cartridge belt.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.