One who purposely wrecks boat sure must change (8)
I believe the answer is:
saboteur
'one who purposely wrecks' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'boat sure must change' is the wordplay.
'must change' is an anagram indicator (I've seen this in other clues).
'boat'+'sure'='boatsure'
'boatsure' is an anagram of 'SABOTEUR'.
Can you help me to learn more?
(Other definitions for saboteur that I've seen before include "One deliberately damages" , "Abort use (anag)" , "Abuser to become a kind of destroyer" , "He destroys in order to subvert" , "Oen who causes industrial damage for pay" .)