Scenario mostly involved poison (7)
I believe the answer is:
arsenic
'poison' is the definition.
(arsenic can be used as a poison)
'scenario mostly involved' is the wordplay.
'mostly' means to remove the last letter.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'scenario' with its last letter taken away is 'scenari'.
'scenari' is an anagram of 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Can rise result from poison?" , "Toxic metallic element used as rat poison" , "Toxic metalloid element" , "Famous stage comedy ''. . . . . . . and Old Lace''" , "Poison used in pesticides and weedkillers" .)