Collapse, give way (4,2)
I believe the answer is:
fall in
I believe this clue is a double definition.
'collapse' is the first definition.
(I know that collapse can be written as fall in)
'give way' is the second definition.
(synonyms)
(Other definitions for fall in that I've seen before include "Army order to form a line - don't collapse!" , "Military order - take place in formation" , "Order to troops to take place in formation" , "Military command to take place in formation" , "'Take one's place in a military line (4,2)'" .)