Tough husband aboard boat (4)
I believe the answer is:
thug
'tough' is the definition.
(both can mean a violent person)
'husband aboard boat' is the wordplay.
'husband' becomes 'h' (abbreviation).
'aboard' is an insertion indicator.
'boat' becomes 'tug' (tug is a kind of boat).
'h' put into 'tug' is 'THUG'.
(Other definitions for thug that I've seen before include "Violent ruffian" , "violent man" , "Hooligan" , "undesirable character" , "Rough" .)