Joined the line and collapsed (4,2)
I believe the answer is:
fell in
This clue is a double definition.
'joined the line' is the first definition.
(to fall in is to join a line of people)
'collapsed' is the second definition.
(I've seen this before)
'and' is the link.
(Other definitions for fell in that I've seen before include "formed a military group" , "collapsed" , "Took one's position" .)