Toxin scare in building (7)
I believe the answer is:
arsenic
'toxin' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'scare in building' is the wordplay.
'building' indicates an anagram.
'scare'+'in'='scarein'
'scarein' is an anagram of 'ARSENIC'.
Can you help me to learn more?
(Other definitions for arsenic that I've seen before include "Poisons in scare" , "killer" , "Poison used in pesticides and weedkillers" , "Brittle grey-green semi-metal, atomic number 33" , "Toxic metallic element used as rat poison" .)