Boat user at sea, a wrecker (8)
I believe the answer is:
saboteur
'a wrecker' is the definition.
('saboteur' can be a synonym of 'wrecker')
'boat user at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'boat'+'user'='boatuser'
'boatuser' is an anagram of 'SABOTEUR'.
(Other definitions for saboteur that I've seen before include "One who disrupts or damages by design" , "Rue boast about one who inflicts deliberate damage" , "One who causes deliberate damage to plants or machinery" , "One deliberately damaging equipment etc" , "Vandal" .)