The most common and most likely answer to this crossword clue is the 6 letter word WOLVES
I've seen this crossword clue in The Guardian
W
O
L
V
E
S
Buddy explains!

The straight part
We know that "Pack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pack = wolves.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 WOLVES mean?
WOLVES noun- The gray wolf, specifically all subspecies of the gray wolf (Canis lupus) that are not dingoes or dogs.
- A man who makes amorous advances to many women.
- A wolf tone or wolf note.
- Any very ravenous, rapacious, or destructive person or thing; especially, want; starvation.
- One of the destructive, and usually hairy, larvae of several species of beetles and grain moths.
- A white worm, or maggot, which infests granaries.
- An eating ulcer or sore. See lupus.
- A willying machine.
- To behave like a wolf.
- Of an organ, to make a hollow whining sound like that of a wolf.