Vessel of some relative interest (4)
I believe the answer is:
vein
'vessel' is the definition.
(vein is a kind of vessel)
'some relative interest' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'VEIN' is hidden in the letters of 'relative interest'.
'of' is the link.
(Other definitions for vein that I've seen before include "part of body" , "Distinctive quality" , "part of a vital system" , "Rock fissure or minerals therein" , "Tube that carries blood to the heart" .)