Missile power sailor unleashed (7)
I believe the answer is:
polaris
'missile' is the definition.
(I've seen this before)
'power sailor unleashed' is the wordplay.
'power' becomes 'p'.
'unleashed' is an anagram indicator (letters are unleashed from their original order).
'sailor' with letters rearranged gives 'olaris'.
'p'+'olaris'='POLARIS'
(Other definitions for polaris that I've seen before include "weapon sailors unleash" , "North Star - missile" , "star (non-movie)" , "Star useful for navigation" , "one belonging in the heavens" .)