A boat that's not so heavy (7)
I believe the answer is:
lighter
This clue is a double definition.
'a boat' is the first definition.
(I know that lighter is a type of boat)
'not so heavy' is the second definition.
('light' can be the opposite of 'heavy' so perhaps 'lighter' is reasonable for 'not so heavy')
'that's' acts as a link.
(Other definitions for lighter that I've seen before include "Less profound" , "Barge - alternative to match" , "Vessel" , "Less weighty" , "Boat for heavy loads and fire igniter" .)