A boat that's not so heavy (7)
I believe the answer is:
lighter
I believe this clue is a double definition.
'a boat' is the first definition.
(lighter is a kind 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' is the 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" .)