Missile's power staggered sailor (7)
I believe the answer is:
polaris
'missile's' is the definition.
(thesaurus)
'power staggered sailor' is the wordplay.
'power' becomes 'p'.
'staggered' indicates anagramming the letters.
'sailor' is an anagram of 'olaris'.
'p'+'olaris'='POLARIS'
(Other definitions for polaris that I've seen before include "weapon sailors unleash" , "North Star - missile" , "Star useful for navigation" , "one belonging in the heavens" , "Star near the N pole of the heavens" .)