Boat that’s not so heavy (7)
I believe the answer is:
lighter
This clue is a double definition.
'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' is the link.
(Other definitions for lighter that I've seen before include "Barge; one needed by smoker" , "craft" , "Goods barge" , "Smoking accessory" , "Fire-raiser" .)