Submarine-launched missile (7)
I believe the answer is:
polaris
'submarine-launched missile' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for polaris that I've seen before include "one belonging in the heavens" , "North Star - missile" , "Star useful for navigation" , "Star near the N pole of the heavens" , "weapon sailors unleash" .)