Condiment makes many sick (4)
I believe the answer is:
dill
'condiment' is the definition.
(thesaurus)
'many sick' is the wordplay.
'many' becomes 'd' (Roman numeral for 500).
'sick' becomes 'ill' (synonyms).
'd'+'ill'='DILL'
'makes' is the link.
(Other definitions for dill that I've seen before include "Herb used in pickles" , "Herb with feathery leaves and spicy seeds" , "Pickling herb" , "Medicinal herb" , "Herb commonly used with gravlax" .)