As reactions get wild, taking time out to leave (7)
I believe the answer is:
arsenic
'as' is the definition.
(synonyms)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for arsenic that I've seen before include "Famous stage comedy ''. . . . . . . and Old Lace''" , "Toxic metalloid element" , "-- and Old Lace (play)" , "Brittle grey-green semi-metal, atomic number 33" , "Powerful poison" .)