Foreign boat sure to be a destroyer (8)
I believe the answer is:
saboteur
'a destroyer' is the definition.
(a saboteur destroys something)
'foreign boat sure' is the wordplay.
'foreign' indicates anagramming the letters (foreign order of the letters).
'boat'+'sure'='boatsure'
'boatsure' anagrammed gives 'SABOTEUR'.
'to be' is the link.
(Other definitions for saboteur that I've seen before include "Underminer, wrecker" , "Rue boast about one who commits industrial espionage" , "One causes damage for political reasons" , "One causing deliberate destruction" , "Hacker possibly" .)