Plot involves arsenic with oxygen (8)
I believe the answer is:
scenario
'plot' is the definition.
(I've seen this before)
'involves arsenic with oxygen' is the wordplay.
'involves' is an anagram indicator.
'with' says to put letters next to each other.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'arsenic' is an anagram of 'scenari'.
'scenari'+'o'='SCENARIO'
(Other definitions for scenario that I've seen before include "In plot of movie, for instance" , "Dramatic situation" , "Storyline" , "Summary of plot of play" , "Imagined situation" .)