Definitions for fall in
collapse, fall in, cave in, give, give way, break, founder(verb)
break down, literally or metaphorically
"The wall collapsed"; "The business collapsed"; "The dam broke"; "The roof collapsed"; "The wall gave in"; "The roof finally gave under the weight of the ice"
to take one's place in a military formation or line
"Troops fall in!"
join, fall in, get together(verb)
become part of; become a member of a group or organization
"He joined the Communist Party as a young man"
To collapse inwards.
Of a soldier, to get into position in a rank.