WordWeb Online

1. Break down, literally or metaphorically
   "The wall fell in";
   - collapse, cave in, give, give way, break, founder
    
2. To take one's place in a military formation or line
   "Troops fall in!"
    
3. Become part of; become a member of a group or organization
   "He fell in the Communist Party as a young man";
   - join, get together

Derived forms: fallen in, falling in, falls in, fell in

Type of: change