Unruly boat user is one causing damage (8)
I believe the answer is:
saboteur
'one causing damage' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'unruly boat user' is the wordplay.
'unruly' is an anagram indicator.
'boat'+'user'='boatuser'
'boatuser' anagrammed gives 'SABOTEUR'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for saboteur that I've seen before include "One causes damage for political advantage" , "One causing deliberate destruction" , "Abuser to become a kind of destroyer" , "Oen who causes industrial damage for pay" , "One causes damage to subvert" .)