Knife, perhaps, in boat (6)
I believe the answer is:
cutter
I believe this clue is a double definition.
'knife perhaps' is the first definition.
(knife is a kind of cutter)
'boat' is the second definition.
(I know that cutter is a type of boat)
'in' is the link.
(Other definitions for cutter that I've seen before include "Fast patrol boat" , "motor launch" , "Knife etc - sailing ship" , "See 1 down" , "One working with Ruby possibly" .)