Flavouring that makes many unwell? (4)
I believe the answer is:
dill
'flavouring' is the definition.
(dill is a kind of flavouring)
'many unwell?' is the wordplay.
'many' becomes 'd' (Roman numeral for 500).
'unwell?' becomes 'ill' (similar in meaning).
'd'+'ill'='DILL'
'that makes' acts as a link.
(Other definitions for dill that I've seen before include "Herb used in cooking" , "Herb of parsley family" , "Herb with thread-like foliage used as seasoning" , "medicinal plant" , "Aromatic herb used as seasoning" .)