The most common and most likely answer to this crossword clue is the 9 letter word STARLINGS
I've seen this crossword clue in The Daily Telegraph
S
T
A
R
L
I
N
G
S
More clues leading to the result STARLINGS
Buddy explains!

The straight part
We know that "they flock together" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they flock together = starlings.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 STARLINGS mean?
STARLINGS noun- A family, Sturnidae, of passerine birds.
- A structure of pilings that protects the piers of a bridge.
- A California fish, the rock trout, Hexagrammos, especially, Hexagrammos decagrammus, the boregat or bodieron.