Destroyer worried boat user (8)
I believe the answer is:
saboteur
'destroyer' is the definition.
(saboteur is a kind of destroyer)
'worried boat user' is the wordplay.
'worried' is an anagram indicator.
'boat'+'user'='boatuser'
'boatuser' anagrammed gives 'SABOTEUR'.
(Other definitions for saboteur that I've seen before include "Wrecker -- sour beat (anag)" , "Rebel" , "One deliberately damages" , "Abuser to become a kind of destroyer" , "One who causes deliberate damage to plants or machinery" .)