Eccentric races in with poisonous substance (7)

I believe the answer is:

arsenic

I'm a little stuck... Click here to teach me more about this clue!

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

'eccentric races in' is the wordplay.
'eccentric' indicates anagramming the letters.
'races'+'in'='racesin'
'racesin' anagrammed gives 'ARSENIC'.

'with' is the link.

Can you help me to learn more?

(Other definitions for arsenic that I've seen before include ", it goes with old lace" , "As briefly identified" , "Toxic metallic element used as rat poison" , "Poisons in scare" , "Famous stage comedy ''. . . . . . . and Old Lace''" .)

I've seen this clue in The Daily Express.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.