Collapse, give way (4,2)
I believe the answer is:
fall in
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">collapse</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that collapse can be written as fall in)</span><br/ ><br/ >'<span class="explanation-format__original">give way</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(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)'" .)