Poison can rise when dissolved (7)
I believe the answer is:
arsenic
'poison' is the definition.
(arsenic can be used as a poison)
'can rise when dissolved' is the wordplay.
'when dissolved' is an anagram indicator.
'can'+'rise'='canrise'
'canrise' is an anagram of 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Element with many poisonous compounds" , "-- and Old Lace (play)" , "Poison used in pesticides and weedkillers" , "killer" , ", it goes with old lace" .)