Tough husband taken on by harbour vessel (4)
I believe the answer is:
thug
'tough' is the definition.
(both can mean a violent person)
'husband taken on by harbour vessel' is the wordplay.
'husband' becomes 'h'.
'taken on by' indicates putting letters inside (inserted letters get taken on or taken in).
'harbour vessel' becomes 'tug' (tugboat).
'h' inserted inside 'tug' is 'THUG'.
(Other definitions for thug that I've seen before include "16 21 [INDIAN IMPORT]" , "Gorilla" , "Violent hoodlum" , "Burrowing crustacean" , "violent gang member" .)