Disorderly retreat (4)
I believe the answer is:
rout
'disorderly retreat' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rout that I've seen before include "Inflict heavy defeat on" , "Decisive defeat" , "Put to flight, disperse" , "Overwhelming victory" , "Overwhelming defeat or disorderly retreat" .)