Destructive migratory insect (6)
I believe the answer is:
locust
'destructive migratory insect' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for locust that I've seen before include "Large insect" , "Destructive winged insect" , "Grasshopper of warm regions" , "Migratory crop stripper" , "Sort of migratory grasshopper" .)