Hot in boat? Tough! (4)
I believe the answer is:
thug
'tough' is the definition.
(both can mean a violent person)
'hot in boat?' is the wordplay.
'hot' becomes 'h' (abbreviation, eg on taps).
'in' means one lot of letters goes inside another.
'boat?' becomes 'tug' (tug is a kind of boat).
'h' inserted within 'tug' is 'THUG'.
(Other definitions for thug that I've seen before include "Violent criminal" , "Bully" , "Brutal criminal" , "A vicious or brutal ruffian" , "Violent lout" .)