The most common and most likely answer to this crossword clue is the 12 letter word TRENDSETTERS
I've seen this crossword clue in The Daily Telegraph
T
R
E
N
D
S
E
T
T
E
R
S
Buddy explains!
The straight part
We know that "leaders of the pack?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case leaders of the pack? = trendsetters.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 TRENDSETTERS mean?
TRENDSETTERS noun- Someone who starts a trend, or makes one more popular