The most common and most likely answer to this crossword clue is the 10 letter word CONFORMERS
I've seen this crossword clue in The Daily Telegraph
C
O
N
F
O
R
M
E
R
S
Buddy explains!
The straight part
We know that "they fall in line" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they fall in line = conformers.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 CONFORMERS mean?
CONFORMERS noun- Any of a set of stereoisomers characterised by a conformation that corresponds to a distinct potential energy minimum.
- A particular folded state or conformation of a protein, especially an abnormal conformation of a prion
- A person who conforms; a conformist.
- A Conformist.