Rush to get a missile (4)
I believe the answer is:
dart
I believe this clue is a double definition.
'rush' is the first definition.
(darting is a kind of rushing)
'a missile' is the second definition.
(dart is a kind of missile)
'to get' is the link.
(Other definitions for dart that I've seen before include "Arrow; dash" , "Small arrow" , "Move quickly and suddenly" , "one destined to go on board?" , "Tuck; arrow" .)