What makes ham have harmful ingredient! (5)
I believe the answer is:
venom
'ingredient' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I cannot understand how the remainder of the clue works.
(Other definitions for venom that I've seen before include "Toxic substance" , "Bitter malice" , "Poisonous secretion" , "acrimony" , "bitterness" .)