Aromatic Eurasian plant whose leaves are used for flavouring in pickles (4)
I believe the answer is:
dill
(Other definitions for dill that I've seen before include "Herb commonly used with gravlax" , "Herb used in pickles" , "Medicinal herb" , "Culinary herb" , "Plant of the celery family; leaf is a herb, seeds a spice" .)