Plant makes 1000 sick (4)
I believe the answer is:
mill
'plant' is the definition.
(I know that mill is a type of plant)
'1000 sick' is the wordplay.
'm000' becomes 'm' (Roman numeral).
'sick' becomes 'ill' (synonyms).
'm'+'ill'='MILL'
'makes' is the link.
(Other definitions for mill that I've seen before include "Vacuous" , "Place for grinding flour" , "No fun going through this" , "Grind (flour)" , "Rob < works" .)