As is nacre, surprisingly (7)
I believe the answer is:
arsenic
'as' is the definition.
('arsenic' can be a synonym of 'as')
'is nacre surprisingly' is the wordplay.
'surprisingly' is an anagram indicator.
'is'+'nacre'='isnacre'
'isnacre' is an anagram of 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Poisons in scare" , "As briefly identified" , "Poison found in races" , "Famous stage comedy ''. . . . . . . and Old Lace''" , "Powerful poison" .)