...poison in races, oddly (7)
I believe the answer is:
arsenic
'poison' is the definition.
(arsenic can be used as a poison)
'in races oddly' is the wordplay.
'oddly' is an anagram indicator.
'in'+'races'='inraces'
'inraces' with letters rearranged gives 'ARSENIC'.
(Other definitions for arsenic that I've seen before include "-- and Old Lace (play)" , "Element No 33" , "Brittle grey-green semi-metal, atomic number 33" , "Poison used in pesticides and weedkillers" , "killer" .)