Conveyors of blood belonging to relative in sanatorium (5)
I believe the answer is:
veins
'conveyors of blood' is the definition.
(veins carry blood)
'belonging to relative in sanatorium' is the wordplay.
'belonging to' says the answer is hidden in the clue (I've seen this in other clues).
'VEINS' is hidden within 'relative in sanatorium'.
(Other definitions for veins that I've seen before include "Circulation aids" , "Characteristic styles" , "Blood vessels" , "Layers of ore" .)