Rib of a leaf (4)
I believe the answer is:
vein
'rib of a leaf' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for vein that I've seen before include "Tube that carries blood to the heart" , "Deposit of mineral" , "Speaker's useless" , "Vessel carrying blood towards the heart" , "part of body" .)