Lunatic races in with toxic substance (7)

I believe the answer is:

arsenic

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'toxic substance' is the definition.
'arsenic' can be an answer for 'substance' (arsenic is a kind of substance). I'm unsure of the 'toxic' bit.

'lunatic races in' is the wordplay.
'lunatic' is an anagram indicator.
'races'+'in'='racesin'
'racesin' is an anagram of 'ARSENIC'.

'with' acts as a link.

Can you help me to learn more?

(Other definitions for arsenic that I've seen before include "Poison found in races" , "Famous stage comedy ''. . . . . . . and Old Lace''" , ", it goes with old lace" , "Toxic metalloid element" , "Ratsbane" .)

I've seen this clue in The Daily Express.

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