Toxic element misbehaving in races (7)
I believe the answer is:
arsenic
'toxic element' is the definition.
(I've seen this before)
'misbehaving in races' is the wordplay.
'misbehaving' indicates an anagram.
'in'+'races'='inraces'
'inraces' anagrammed gives 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "Famous stage comedy ''. . . . . . . and Old Lace''" , "Brittle grey-green semi-metal, atomic number 33" , "Toxic metallic element used as rat poison" , "Ratsbane" , "Can rise result from poison?" .)