fall in phrasal verb ( see also ↑ fall )
1 . if the roof, ceiling etc falls in, it falls onto the ground SYN collapse
2 . to start walking or forming a line of people behind someone else
fall in behind
His men fell in behind him.
FALL IN
Meaning of FALL IN in English
Longman Dictionary of Contemporary English. Longman - Словарь современного английского языка. 2012