fall in

verb5

1. •
   To collapse inwards.

   • The heavy rain caused the roof to fall in.
2. •
   Of a soldier, to get into position in a rank.

   • His men resumed their muskets and fell in. — 1860
3. •
   To take a position.

   • At that point, the Subaru fell in behind the Toyota.
4. •
   To come to an end; to terminate; to lapse.

   • On the death of Mr. B., the annuity, which he had so long received, fell in.
5. •
   To become operative.

   • The reversion, to which he had been nominated twenty years before, fell in. — 1841