Vessel said to be light (4)
I believe the answer is:
pail
'vessel' is the definition.
(I know that pail is a type of vessel)
'said to be light' is the wordplay.
'said to be' shows a homophone (sound like).
'light' becomes 'pale' (similar in meaning).
'pale' is a homophone of 'PAIL'.
(Other definitions for pail that I've seen before include "Milkmaid's bucket?" , "water-carrier" , "Vessel for carrying liquids" , "Kind of bucket - ask Jack and Jill" , "Cylindrical vessel" .)