A flavouring that makes many sick? (4)
I believe the answer is:
dill
'a 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' is the link.
(Other definitions for dill that I've seen before include "Herb used in pickles" , "medicinal plant" , "Medicinal herb" , "Herb of parsley family" , "Plant of the celery family; leaf is a herb, seeds a spice" .)