Vessel is worthless, they say (4)
I believe the answer is:
vein
'vessel' is the definition.
(I know that vein is a type of blood vessel)
'worthless they say' is the wordplay.
'worthless' becomes 'vain' (I've seen this before).
'they say' shows a homophone (sound like).
'vain' is a homophone of 'VEIN'.
'is' is the link.
(Other definitions for vein that I've seen before include "Distinctive style" , "Vessel for claret" , "Blood-carrying vessel" , "Leaf rib; ore layer in rock" , "Rock fissure or minerals therein" .)