The most common and most likely answer to this crossword clue is the 4 letter word MOBS
I've seen this crossword clue in The Times
M
O
B
S
Buddy explains!

The straight part
We know that "Falls on" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case falls on = mobs.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 MOBS mean?
MOBS noun- A large or disorderly group of people; especially one bent on riotous or destructive action.
- A group of animals such as horses or cattle.
- A flock of emus.
- The Mafia, or a similar group that engages in organized crime (preceded by the).
- A non-player character, especially one that exists to be fought or killed to further the progression of the story or game.
- The lower classes of a community; the rabble.
- (Australian Aboriginal) A group of Aboriginal people associated with an extended family group, clan group or wider community group, from a particular place or country.
- To crowd around (someone), sometimes with hostility.
- To crowd into or around a place.
- A promiscuous woman; a harlot or wench; a prostitute.
- A mob cap.
- To wrap up in, or cover with, a cowl.
- Mobile phone