Dock denizen (3)
I believe the answer is:
rat
'dock denizen' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rat that I've seen before include "Betrayer" , "Treacherous person" , "one escaping doomed vessel?" , "Rodent pest" , "that gets out of stricken vessel" .)