Arsonist in boat? (7)
I believe the answer is:
lighter
This clue is a double definition.
'arsonist' is the first definition.
'boat' is the second definition.
(I know that lighter is a type of boat)
'in' acts as a link.
(Other definitions for lighter that I've seen before include "carrying less weight" , "Substitute for match" , "The girl (anag.)" , "Barge; igniter" , "Canal boat with a flat bottom" .)