Flavouring that makes many sick? (4)
I believe the answer is:
dill
'flavouring' is the definition.
(dill is a kind of flavouring)
'many sick?' is the wordplay.
'many' becomes 'd' ().
'sick?' becomes 'ill' ('ill' can be a synonym of 'sick').
'd'+'ill'='DILL'
'that makes' acts as a link.
(Other definitions for dill that I've seen before include "Aromatic herb used as seasoning" , "Pickling herb" , "Herb commonly used with gravlax" , "Herb; rhymes with beak" , "Plant of the celery family; leaf is a herb, seeds a spice" .)