The most common and most likely answer to this crossword clue is the 6 letter word KITBAG
I've seen this crossword clue in The Daily Telegraph
K
I
T
B
A
G
More clues leading to the result KITBAG
Buddy explains!

The straight part
We know that "Knapsack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knapsack = kitbag.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 KITBAG mean?
KITBAG noun- A large cylindrical holdall.