Rush to get missile (4)
I believe the answer is:
dart
This clue is a double definition.
'rush' is the first definition.
(darting is a kind of rushing)
'missile' is the second definition.
(dart is a kind of missile)
'to get' acts as a link.
(Other definitions for dart that I've seen before include "Move quickly and suddenly" , "Small arrow" , "flash" , "Shoot out" , "Arrow - go fast" .)