The most common and most likely answer to this crossword clue is the 8 plus 3 letter word SLEEPING BAG
I've seen this crossword clue in The Daily Telegraph
S
L
E
E
P
I
N
G
B
A
G
More clues leading to the result SLEEPING BAG
Buddy explains!
The straight part
We know that "Nap sack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nap sack = sleeping bag.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.