Herb that makes many unwell (4)
I believe the answer is:
dill
'herb' is the definition.
(dill is a kind of herb)
'many unwell' is the wordplay.
'many' becomes 'd' ().
'unwell' becomes 'i'll' (similar in meaning).
'd'+'ill'='DILL'
'that makes' acts as a link.
(Other definitions for dill that I've seen before include "Herb; rhymes with beak" , "Herb of parsley family" , "Herb for gravadlax" , "Herb with thread-like foliage used as seasoning" , "Culinary herb" .)