Leaf line (4)
I believe the answer is:
vein
'leaf line' 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 "Rock fissure or minerals therein" , "Part of the blood circulation system" , "Speaker's useless" , "part of body" , "Leaf rib; ore layer in rock" .)